Powder coating of epoxy-acrylic resin, polycarboxylic acid, crosslinked particles and liquid resin

ABSTRACT

A thermosetting powder coating composition prepared by a wet process, which contains a mixture comprising (a) an epoxy-containing acrylic resin, (b) a polycarboxylic acid compound curing agent, (c) fine crosslinked resin particles and/or (d) a resin that exists in the liquid form at room temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to thermosetting powder coatingcompositions and curing agent compositions, and more particularly tothermosetting powder coating compositions excellent in storage stabilityand capable of providing coating films with excellent appearance andalso to curing agent compositions for use in the thermosetting powdercoating compositions.

2. Description of Related Art

Powder coatings which do not release organic solvents to an atmospherehave been widely noted as eco-friendly coatings.

Currently, the use of thermosetting powder coatings is popular in thepowder coating field for their ability to provide coating filmsexcellent in performances and physical properties. For their applicationto automotive bodies, the improvements in smoothness and appearance ofresulting films have been demanded. One technique used to improve thefilm smoothness is the thick application of powder coatings. Anothertechnique is the use of powder coatings prepared from the materialshaving low melt viscosity. While effective in achieving someimprovements in smoothness and appearance of resulting films onautomobile horizontal portions such as a hood and the like, thesetechniques have suffered from the problem. That is, the powder coatingsapplied onto automobile vertical portions such as doors and the like,when subsequently heated and melted, result in coating films which showa marked reduction in smoothness and appearance as a result of theoccurrence of sagging.

The incorporation of fine crosslinked resin particles, called microgels,to prevent the occurrence of sagging is known in the solvent-bornecoating field. For example, Japanese Patent Laying-Open Nos. Sho49-97026 (1974) and Sho 60-250068 (1985) disclose techniques whichcontemplate to reduce sagging and accordingly improve film appearance byadding microgels to top coatings.

In the powder coating field, the use of powder coatings containing finecrosslinked resin particles is disclosed, for example, in JapanesePatent Laying-Open Nos. Sho 62-97026 (1987). However, such powdercoatings are prepared by mixing dry fine particles of crosslinked resinunder a solvent-free condition. During the mixing, the crosslinked resinparticles are often caused to locally form agglomerates. This disturbsuniform distribution thereof to result in the unsatisfactory smoothnessand appearance of obtaining coating films. In the case where a meltmixing process is added to precede the dry mixing process, a majority offine particles of crosslinked resin are caused to deposit on surfaces ofresulting powder coating particles. When such powder coating particlesare applied and subsequently heated, the interaction is caused to occurbetween the fine crosslinked resin particles. This has led often to theinsufficient smoothness and appearance of resulting coating films.

Powder coatings, when heated to melt, result in coating films. However,such coating films show the insufficient smoothness compared to thoseobtained from solvent-borne coatings. In order to improve the smoothnessof resulting coating films, the melt viscosities of components containedin the powder coatings, i.e., the melt viscosity of raw material must bereduced to improve the fluidity thereof during melt.

One approach to lower the melt viscosity of raw material may be the useof low-melting point or low-molecular-weight substances for the rawmaterial. While effective in improving the smoothness of resultingfilms, their use has also led to the reduction in storage properties,such as resistance to blocking and solid reaction. The difficulty hasthus been to reconcile the storage stability of the coating compositionand the smoothness of resulting film.

In Japanese Patent Laying-Open No. Hei 9-100414 (1997), a method isdisclosed which produces thermosetting resin particles having a narrowparticle size distribution by utilizing a wet process. Since thethermosetting resin particles are produced in an aqueous medium, theiruse has led to the improvement in one of storage properties, i.e.,resistance to solid reaction. It has been unsatisfactory, however, toreconcile the blocking resistance of the powder coating composition andthe smoothness of the obtaining film.

Among thermosetting powder coatings, acrylic-based coatings, when formedinto films, provide excellent performances and physical properties. Theresulting films however suffer from a problem of poor appearance, calleda hazing phenomenon. This phenomenon is known to often occur when alarge amount of polybasic acid is used as a curing agent. If the reducedamount of polybasic acid is used to suppress the occurrence of such ahazing phenomenon, the performances and physical properties of resultingfilms then become insufficient. Also, there exists no polybasic acidwhich can act to improve performances and physical properties ofresulting films while controlling the occurrence of the hazingphenomenon, so far as we know.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a thermosettingpowder coating composition which, when applied and then heated, shows nooccurrence of sagging to result in coating films with satisfactorysmoothness and appearance.

It is a second object of the present invention to provide athermosetting powder coating composition which has an excellent storagestability and can be formed into films with excellent smoothness.

It is a third object of the present invention to provide a curing agentcomposition which, when used as a curing agent for acrylic based powdercoatings, serves to provide films which show no occurrence of hazingphenomenon, and also to provide a method of producing the curing agentcomposition.

In accordance with a first aspect of the invention, a thermosettingpowder coating composition is provided which is prepared by a wetprocess from a formulation containing an epoxy-containing acrylic resin(a), a polycarboxylic acid compound curing agent (b) and finecrosslinked resin particles (c).

The above-stated wet process preferably includes the following steps:

1. The above-described formulation components are allowed to dissolve ordisperse in an organic solvent to provide a raw material solution.

2. The raw material solution obtained in step 1 is added to an aqueoussolution containing a water-soluble polymer that has a cloud pointwithin the temperature range of 30-90° C. and mix them at a temperaturebelow the cloud point to thereby prepare a suspension containing primaryoil particles.

3. The suspension obtained in step 2 was heated to a temperature equalto or above the cloud point to form secondary oil particles and theorganic solvent was distilled off to collect particles.

The aforementioned epoxy-containing acrylic resin (a) contains anepoxy-containing acrylic resin A and an epoxy-containing acrylic resinB. These acrylic resins A and B preferably satisfy the followingrelationships:

(1) (SP_(A)-SP_(B)) is within the range of 0.2-1.5, whereSP_(A)=solubility parameter of resin A and SP_(B)=solubility parameterof resin B;

(2) Tg(A)-Tg(B)≧10° C., where Tg(A)=glass transition temperature ofresin A and Tg(B)=glass transition temperature of resin B;

(3) Tg(A) is within the range of 40-100° C. and Tg(B) is within therange of 20-50° C.; and

(4) A ratio in solid weight of resin A to B is within the range of5/95-50/50.

The aforementioned formulation may further contain a resin (d) thatexists in the liquid form at room temperature and preferably satisfiesthe following relationships:

(1) (SP_(B)-SP_(d)) is within the range of 0.01-1.5, where SP_(d) issolubility parameter of resin (d); and

(2) A solids weight of the resin (d) is 5-70 parts, based on 100 partsof a total solids weight of the epoxy-containing acrylic resin (a) andresin (d).

In accordance with a second aspect of the present invention, athermosetting powder coating composition is provided which is preparedby a wet process from a formulation containing an epoxy-containingacrylic resin (a), a polycarboxylic acid compound curing agent (b) and aresin (d) that exists in the liquid form at room temperature.

The above-stated wet process preferably includes the following steps:

1. A raw material solution is formed by allowing the above-describedformulation to dissolve or disperse in an organic solvent.

2. The raw material solution obtained in step 1 is mixed with an aqueoussolution containing a water-soluble polymer which shows a cloud point ata temperature within the range of 30-90° C. to prepare a suspensioncontaining primary oil particles.

3. The suspension obtained in step 2 was heated to a temperature of notbelow the cloud point to prepare secondary oil particles while removingthe organic solvent to collect particles.

The aforementioned epoxy-containing acrylic resin (a) may contain anepoxy-containing acrylic resin A and another epoxy-containing acrylicresin B. The formulation components preferably satisfies the followingrelationships:

(1) A number average molecular weight of resin A is in the range of2,000-4,000;

(2) Tg(A) is within the range of 40-100° C., where Tg(A)=glasstransition temperature of resin A;

(3) Tg(A)-Tg(B)≧10° C., where Tg(B)=glass transition temperature ofresin B;

(4) (SP_(A)-SP_(B)) is within the range of 0.2-1.5, whereSP_(A)=solubility parameter of resin A and SP_(B)=solubility parameterof resin B;

(5) A solids amount of resin B is 50-95% by weight of the total solidsof epoxy-containing acrylic resin (a);

(6) (SP_(B)-SP_(d)) is within the range of 0.01-1.5, whereSP_(d)=solubility parameter of resin (d); and

(7) The solids amount of resin (d) is 5-70 parts by weight, based on 100parts by weight of the total solids of the aforementionedepoxy-containing acrylic resin (a) and resin (d).

In accordance with a third aspect of the invention, a curing agentcomposition (b) is provided which contains a polycarboxylic acidcompound (b-1) that exists in the crystalline solid form at roomtemperature, and a carboxylic acid compound (b-2) dissimilar in type tothe polycarboxylic acid compound (b-1). Characteristically, the curingagent composition satisfies the following relationships:

(1) The curing agent composition (b) exists in the solid form at roomtemperature; and

(2) A melting point of the curing agent composition (b), as determinedby DSC (differential scanning calorimeter), is lower than a meltingpoint of the polycarboxylic acid compound (b-1) or the carboxylic acidcompound (b-2).

The melting point of the curing agent composition (b) is preferablywithin the 60-180° C. range.

Preferably, the melting point of the curing agent composition (b) islower than the melting point of the polycarboxylic acid compound (b-1)and lower than the melting point of the carboxylic acid compound (b-2).

Preferably, a ratio in weight of the polycarboxylic acid compound (b-1)to carboxylic acid compound (b-2) is 50:50-99:1.

A multilayer film-forming method of the present invention includes thesteps of:

applying a basecoat onto an undercoated and optionally intercoatedsubstrate;

applying the powder coating composition of the present invention ontothe basecoat; and

heating the substrate carrying thereon the basecoat and powder coatinglayers.

A multilayer film of the present invention is the multilayer filmprepared according to the above-described multilayer film-formingmethod.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing an illustrative DSC absorption curve from thecuring agent composition of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The term “room temperature”, as used herein, refers to 25° C. The “solidform” means the state wherein a substance has definite volume andconfiguration. The “liquid form” means the state wherein a substance hasa definite volume but does not have a definite configuration.

Thermosetting Powder Coating Composition

The thermosetting powder coating composition according to the firstaspect of the invention is prepared by a wet process and contains, asraw material, an epoxy-containing acrylic resin (a), a polycarboxylicacid compound curing agent (b) and fine crosslinked resin particles (c).

The thermosetting powder coating composition according to the secondaspect of the invention is prepared by a wet process and contains, asraw material, an epoxy-containing acrylic resin (a), a polycarboxylicacid compound curing agent (b) and a resin (d) that exists in the liquidform at room temperature.

Epoxy-Containing Acrylic Resin (a)

The epoxy-containing acrylic resin (a) incorporated in the thermosettingpowder coating composition of the present invention is not particularlyspecified in type, and its specific example is a resin prepared bypolymerizing at least one type of epoxy-containing monomer, optionallywith other type of monomer that does not undergo a reaction with anepoxy group of the epoxy-containing monomer, according to a conventionalprocedure. Examples of epoxy-containing monomers include glycidyl(meth)acrylate, 2-methyl glycidyl methacrylate and the like. Examples ofoptional monomers which do not react with an epoxy group of theepoxy-containing monomer include hydroxyl-containing monomers such as2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,4-hydroxybutyl acrylate, addition products (product name: PLAXEL FMSeries, manufactured by Daicel Chem. Ind. Co., Ltd.) of 2-hydroxyethyl(meth)acrylate and polycaprolactone and polyalkylene glycolmono(meth)acrylates; and neutral monomers such as methyl (meth)acrylate,ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate,tert-butyl (meth)acrylate, styrene, vinyl toluene, p-chloro styrene andthe like. The above-listed epoxy-containing monomers may be used aloneor in any combination thereof. Similarly, the above-listed optionalmonomers may be used alone or in any combination thereof.

The aforementioned epoxy-containing acrylic resin (a) is preferablypresent in the solid form at room temperature.

The SP (solubility parameter) value of the epoxy-containing acrylicresin (a) is not particularly specified, but is generally in the rangeof 9.0-12.0, preferably in the range of 9.0-11.0, more preferably in therange of 9.5-11.0. The SP values can be determined by a method known tothose skilled in the art, such as a turbidimetry method.

The Tg (glass transition temperature) of the epoxy-containing acrylicresin (a) is not particularly specified, but is preferably in the rangeof 20-100° C. Tg's can be determined by a differential scanningcalorimeter. Alternatively, Tg's can be calculated from simultaneousequations including a given condition on a ratio of comonomeric unitshaving known Tg's.

The epoxy-containing acrylic resin (a) may comprise one or more types ofepoxy-containing acrylic resins. For example, a thermosetting powdercoating composition which provides coating films having improvedblocking resistance and smoothness can be obtained if theepoxy-containing acrylic resin (a) comprises two different types ofepoxy-containing acrylic resins A and B which preferably satisfy thefollowing relationships:

(1) (SP_(A)-SP_(B)) is within the range of 0.2-1.5, whereSP_(A)=solubility parameter of resin A and SP_(B)=solubility parameterof resin B;

(2) Tg(A)-Tg(B)≧10° C., where Tg(A)=glass transition temperature ofresin A and Tg(B)=glass transition temperature of resin B;

(3) Tg(A) is within the range of 40-100° C. and Tg(B) is within therange of 20-50° C.; and

(4) A ratio in solids weight of resin A to B is within the range of5/95-50/50.

The aforementioned resins A and B may or may not be reactive with eachother to undergo a curing reaction when heated. Since the thermosettingpowder coating composition of the present invention further includes thebelow-described curing agent, even the resins A and B of the lattercase, when heated, are caused to cure via the reaction with the curingagent. This assures sufficient performances of resulting coating films.

If (SP_(A)-SP_(B))<0.2, the blocking resistance of the resulting powdercoatings may deteriorate during storage. If (SP_(A)-SP_(B))>1.5, theresulting powder coatings, when formed into films, may provide the poorappearance.

If Tg(A)-Tg(B)<10° C., the blocking resistance of the resulting powdercoatings may deteriorate during storage. As stated above, Tg(B) ispreferably within the range of 20-50° C. If Tg(B) is lower than 20° C.,the blocking resistance of the resulting powder coatings may deteriorateduring storage. On the other hand, if Tg(B) is higher than 50° C., theresulting powder coatings, when formed into films, may provide the poorsmoothness.

Also, a number average molecular weight of the resin A is preferably inthe range of 2,000-4,000. If it is lower than 2,000, the blockingresistance of the resulting powder coatings may deteriorate duringstorage. On the other hand, if it is higher than 4,000, the resultingpowder coatings, when formed into films, may provide the poorsmoothness.

As also stated above, Tg(A) is preferably within the range of 40-100° C.If Tg(A) is lower than 40° C., the blocking resistance of the resultingpowder coatings may deteriorate during storage. On the other hand, ifTg(A) is higher than 100° C., the resulting powder coatings, when formedinto films, may provide the poor smoothness.

A resin solids of the epoxy-containing acrylic resin (a) for use in thethermosetting powder coating composition generally amounts to 100-1,000epoxy equivalents. For the further increased performances and physicalproperties of resulting films, the epoxy equivalent is preferablyadjusted to fall within the range of 150-600, more preferably within therange of 200-400.

Polycarboxylic Acid Compound Curing Agent (b)

The thermosetting powder coating composition of the present inventionfurther contains a polycarboxylic acid compound curing agent (b). Thepolycarboxylic acid compound curing agent (b) preferably exists in thesolid form at room temperature. While not particularly specified, amelting point of the polycarboxylic acid compound curing agent (b) ispreferably in the range of 60-180° C., more preferably in the range of80-150° C. If the melting point is lower than 60° C., the storagestability of the resulting powder coatings may deteriorate. If it ishigher than 180° C., the resulting powder coatings, when formed intofilms, may provide the unsatisfactory smoothness and appearance.

Representative of the polycarboxylic acid compound curing agent (b)incorporated in the thermosetting powder coating composition of thepresent invention are aliphatic polycarboxylic acid compounds andaromatic polycarboxylic acid compounds. Examples of aliphaticpolycarboxylic acid compounds include decanedicarboxylic acid, adipicacid, maleic acid, malonic acid, ethylmalonic acid, butylmalonic acid,dimethylmalonic acid, succinic acid, methylsuccinic acid,dimethylsuccinic acid, glutaric acid, methylglutaric acid,dimethylglutaric acid, sebacic acid, azelaic acid, pimelic acid, subericacid, 1,11-undecanoic acid, dodecane dicarbbxylic acid, brassilic acid,hexadecane carboxylic acid, 3-iso-octylhexane dicarboxylic acid,cyclohexane dicarboxylic acid, butane tricarboxylic acid, butanetetracarboxylic acid, citric acid, tricarballylic acid and the like.

Examples of aromatic polycarboxylic acid compounds include phthalic acidand the like. Anhydrides thereof can also be used, examples of whichinclude succinic anhydride, tetrahydrophthalic anhydride, phthalicanhydride and the like.

Other than the above-described compounds, synthesized polycarboxylicacid compounds can also be used for the polycarboxylic acid compoundcuring agent (b) of the present invention. Specifically, they may bepolycarboxylic acid compounds, for example, obtained via reactions ofpolyols with acid anhydrides. Examples of such compounds includebutanediol succinate prepared from butanediol and succinic anhydride,hexanediol succinate prepared from hexanediol and succinic anhydride,nonanediol succinate prepared from nonanediol and succinic anhydride,1:1:1 addition product of neopentyl glycol, trimellitic anhydride andsuccinic anhydride.

For the polycarboxylic acid compound curing agent (b), the above-listedpolycarboxylic acid compounds may be used in any combination thereof.

From the viewpoint of film appearance, the polycarboxylic acid compoundcuring agent (b) may preferably comprise the curing agent compositionaccording to the third aspect of the present invention. That is, thepreferred polycarboxylic acid compound curing agent (b) contains apolycarboxylic acid compound (b-1) that exists in the crystalline solidform at room temperature, and a carboxylic acid compound (b-2)dissimilar in type to the polycarboxylic acid compound (b-1), and alsosatisfies the following conditions:

(1) The polycarboxylic acid compound curing agent (b) exits in the solidform at room temperature; and

(2) A melting point of the polycarboxylic acid compound curing agent(b), as determined by a differential scanning calorimeter (hereinafterreferred to as DSC), is lower than a melting point of the polycarboxylicacid compound (b-1) or carboxylic acid compound (b-2).

Among the polycarboxylic acid compounds described above as being presentin the solid form at room temperature, those which exist in thecrystalline solid form at room temperature may be used for thepolycarboxylic acid compound (b-1). For the polycarboxylic acid compound(b-1), such polycarboxylic acid compounds may be used in any combinationthereof.

The carboxylic acid compound (b-2) dissimilar in type to theabove-described polycarboxylic acid compound (b-1) is not particularlyspecified, and may be chosen, for example, from polycarboxylic acidcompounds that exist in the crystalline solid form at room temperature,as referred to in the description of the polycarboxylic acid compound(b-1); polycarboxylic acid compounds that exist in the amorphous solidor liquid form at room temperature; and monocarboxylic acid compoundswhich do not have a definite form at room temperature. Specific examplesof useful carboxylic acid compounds (b-2) may be those which exist inthe liquid form at room temperature, including aliphatic monocarboxylicacid compounds such as lauric acid, stearic acid and 8-ethyloctadecanoicacid; 1:2 addition product of nonanediol and hexahydrophthalic anhydrideand the like. The above-described carboxylic acid compounds (b-2) may beused in any combination thereof.

A melting point of the polycarboxylic acid compound curing agent (b),which contains the polycarboxylic acid compound (b-1) and carboxylicacid compound (b-2), is not particularly specified. For the better filmappearance, it is preferably below a melting point of the polycarboxylicacid compound (b-1) or carboxylic acid compound (b-2). More preferably,it is lower than the respective melting points of the polycarboxylicacid compound (b-1) and carboxylic acid compound (b-2).

For the purposes of this disclosure, respective melting points of thepolycarboxylic acid compound curing agent (b), polycarboxylic acidcompound (b-1) and carboxylic acid compound (b-2) are given byDSC-determined values. The melting point of a selected compound isdetermined by the lowest temperature in a temperature range where a DSCcurve shows an great heat absorption of the selected compound. In thecase where a DSC curve from a certain compound shows a gentle slopeonly, a temperature at which heat absorption is started is determined asits melting point.

DSC absorption curves from the polycarboxylic acid compound (b-1),carboxylic acid compound (b-2) and polycarboxylic acid compound curingagent (b) are illustrated in FIG. 1. FIG. 1 illustrates a DSC absorptioncurve from decanedicarboxylic acid, as representing the polycarboxylicacid compound (b-1), a DSC absorption curve from sebacic acid asrepresenting the carboxylic acid compound (b-2), and a DSC absorptioncurve from the compound which contains decanedicarboxylic acid andsebacic acid in the ratio by weight of 50:50, as representing thepolycarboxylic acid compound curing agent (b).

In the present invention, for the above-specified compounds, the meltingpoint of each compound is determined by the lowest temperature in atemperature range where its DSC absorption curve shows an great heatabsorption. In the case where a DSC absorption curve for any of thosecompounds shows a gentle slope only, the temperature at which heatabsorption is started is determined as its melting point. For example,the melting point of the polycarboxylic acid compound (b-1) can bedetermined as 125° C., the melting point of the carboxylic acid compound(b-2) as 130° C., and the melting point of the polycarboxylic acidcompound curing agent (b) as 110° C., respectively from the DSCabsorption curves depicted in FIG. 1. In the present invention, it isimportant to find relative relationships between the compounds in termsof a melting point. Accordingly, the values of the determined meltingpoints are permitted to include slight errors, if not significant.

As can be appreciated from illustrative DSC absorption curves depictedin FIG. 1, the DSC absorption curve from the polycarboxylic acidcompound curing agent (b) is identical to neither of the DSC absorptioncurves from its component compounds, i.e., the polycarboxylic acidcompound (b-1) and carboxylic acid compound (b-2), and is different fromthe simple composition of the DSC absorption curves from those twocompounds. Also, a melting point of the polycarboxylic acid compoundcuring agent (b) has been found to be lower than that of at least one ofthe polycarboxylic acid compound (b-1) and carboxylic acid compound(b-2). These demonstrate that the state of the polycarboxylic acidcompound curing agent (b) does not reflect a simple mixture of thepolycarboxylic acid compound (b-1) and carboxylic acid compound (b-2).One of the reasons why the polycarboxylic acid compound curing agent (b)exhibits a melting point lower than that of at least one of thepolycarboxylic acid compound (b-1) and carboxylic acid compound (b-2) isconsidered because the higher melting point compound decreases itssurface free energy as the lower melting point compound changes its formto a liquid. Another reason would be as follows: As the atomic vibrationof the lower melting point compound which has changed its form to aliquid becomes intensive, surface atoms of the higher melting pointcompound are also caused to vibrate intensively and finally liquefy.This results in lowering a temperature at which the higher melting pointcompound reaches a critical vibration below which its solid form ismaintained.

Unless the DSC-determined melting point of the polycarboxylic acidcompound curing agent (b) is lower than that of the polycarboxylic acidcompound (b-1) or carboxylic acid compound (b-2), the improvement inappearance of obtaining films may become insufficient.

Preferably, the polycarboxylic acid compound curing agent (b) has amelting point in the range of 60-180° C., and more preferably in therange of 80-150° C. For the better smoothness and appearance ofresulting films, it is further preferred that the polycarboxylic acidcompound curing agent (b) has a lower melting point relative to thepolycarboxylic acid compound (b-1) and carboxylic acid compound (b-2).

The ratio by weight of the polycarboxylic acid compound (b-1) tocarboxylic acid compound (b-2), when incorporated in the polycarboxylicacid compound. curing agent (b), is not particularly specified and maybe suitably chosen by those skilled in the art. Preferably, it is in therange of 50:50-99:1. If the proportion by weight of the polycarboxylicacid compound (b-1) is below 50, a resulting polycarboxylic acid curingagent compound (b) may fail to take a solid form at room temperature. Ifit exceeds 99, the improvement in appearance of obtaining films maybecome insufficient.

A technique used to obtain the polycarboxylic acid compound curing agent(b) preferably involves mixing of the polycarboxylic acid compound (b-1)and carboxylic acid compound (b-2). There are two mixing techniqueswhich can be employed. One technique mixes the polycarboxylic acidcompound (b-1) and carboxylic acid compound (b-2) after particle sizeminiaturization thereof. Another technique mixes them in the liquidform. The latter technique is preferred for its ability to provide amore uniform mixture leading to the improvement in smoothness ofresulting films.

Where the former technique is employed, the particle sizeminiaturization of polycarboxylic acid compound (b-1) and carboxylicacid compound (b-2) is performed such that the resulting mixture, i.e.,the polycarboxylic acid compound curing agent (b) has a volume meanparticle diameter preferably of not exceeding 15 μm, more preferably ofnot exceeding 10 μm.

Comminuting must be carried out to establish the particle sizeminiaturization thereof. The comminution can be performed eitherconcurrently with or prior to mixing. Alternatively, preliminarycomminuting may be done before the simultaneous practice of finalcomminuting and mixing. Such comminuting can be achieved by using aconventionally-known means for grinding or pulverizing solids. A mortarmay be used, for example. From an industrial point of view, such a meansis preferably chosen from comminuting machines generally used in thepowder coating art, such as a Henschel mixer and a sand grinding mill,depending upon the end purpose contemplated. The particle size reductionallows the polycarboxylic acid compound (b-1) and carboxylic acidcompound (b-2) to be mixed sufficiently. When desired to obtain thepolycarboxylic acid compound curing agent (b) in the form of adispersion, the aforementioned comminuting and mixing may be carried outin the presence of a solvent.

The other technique which involves mixing the compounds in the liquidform can be classified into two modes. Mode one involves allowing thepolycarboxylic acid compound (b-1) and carboxylic acid compound (b-2) tomelt simultaneously and then cooling to a temperature sufficient tosolidify them. Mode two involves allowing the polycarboxylic acidcompound (b-1) and carboxylic acid compound (b-2) to dissolve in asuitable solvent and, subsequent to mixing, distilling the solvent offto solidify them. Specific conditions at each stage may be suitablychosen depending upon the particular types of the polycarboxylic acidcompound (b-1) and carboxylic acid compound (b-2) used. The solventsuitable for use in dissolving the compounds is not particularlyspecified. Examples of suitable solvents include aliphatic hydrocarbonssuch as pentane, hexane and heptane; aromatic hydrocarbons such asbenzene, toluene and xylene; alicyclic hydrocarbons such as cyclohexane,methylcyclohexane and decalin; halogenated hydrocarbons such as carbontetrachloride, chloroform and 1,2-dichloroethane; ethers such as ethylether, isopropyl ether, anisole, dioxane and tetrahydrofuran; ketonessuch as acetone, methyl isobutyl ketone, methyl ethyl ketone,cyclohexanone, acetophenone and isophorone; esters such as ethyl acetateand butyl acetate; petroleum ether and petroleum benzine.

The polycarboxylic acid compound curing agent (b) obtained in the manneras described above is then reduced in size by the aforementionedcomminuting means to a volume mean particle diameter preferably of notexceeding 15 μm, more preferably of not exceeding 10 μm.

Various methods known to those skilled in the art, such as an electricresistance method and laser light scattering method, can be utilized todetermine the volume mean particle diameter.

Fine Crosslinked Resin Particles (c)

The thermosetting powder coating composition according to the firstaspect of the present invention contains fine crosslinked resinparticles (c), other than the afore-stated epoxy-containing acrylicresin (a) and polycarboxylic acid compound curing agent (b).

Useful fine crosslinked resin particles (c) are known to those skilledin the art. Examples of fine crosslinked resin particles include polymerparticles which have intraparticle crosslinking bonds and are insolubleto organic solvents. Such polymer particles can be prepared, forexample, by allowing a crosslinkable monomer having two or morefree-radically polymerizable, ethylenically unsaturated groups in amolecule, such as ethylene glycol dimethacrylate or divinylbenzene, topolymerize in the presence of an emulsifying agent such as a polyesterresin containing amphoteric ionic groups, or alternatively, by allowingan acrylic monomer containing two or more monomer components reactivewith each other to form crosslinking bonds, such as a combination of acarboxyl-containing monomer and an epoxy-containing monomer, topolymerize in the presence of an emulsifying agent such as a polyesterresin containing amphoteric ionic groups. Also, the fine crosslinkedresin particles may comprise so-called core-shell type crosslinked resinfine particles which can be obtained via a two-stage crosslinkingreaction.

The fine crosslinked resin particles have a volume mean particlediameter, for example, preferably of 0.01-10 μm, more preferably of0.01-5 μm. However, such a volume mean particle diameter may be suitablyselected depending upon the desired volume mean particle diameter of theresulting thermosetting powder coating composition. If theabove-described volume mean particle diameter is smaller than 0.01 μm,the effect of improving a film appearance may become insufficient. Onthe other hand, if it is larger than 10 μm, the resulting powdercoatings, when formed into films, may provide the poor appearances.

The thermosetting powder coating composition contains the finecrosslinked resin particles (c) preferably in the amount of 0.1-30 partsby weight, more preferably in the amount of 0.1-5 parts by weight, basedon 100 parts by weight of the total solids in the aforementionedepoxy-containing acrylic resin (a) and polycarboxylic acid compoundcuring agent (b). If the amount of fine crosslinked resin particlesincorporated falls below 0.1 parts by weight, the effect of improving afilm appearance may become insufficient. On the other hand, if itexceeds 30 parts by weight, the resulting powder coatings, when formedinto films, may provide the poor appearances.

While not particularly specified, a Tg of fine crosslinked resinparticles (c) is preferably between −20° C. and 110° C., more preferablybetween 0° C. and 110° C. If the Tg is below −20° C., the storagestability of the resulting powder coatings may deteriorate. On the otherhand, if the Tg is above 110° C., the resulting powder coatings, whenformed into films, may provide the poor smootheness.

Resin (d) that Exists in the Liquid Form at Room Temperature

The thermosetting powder coating composition according to the secondaspect of the present invention further contains a resin (d) that existsin the liquid form at room temperature, other than the above-describedepoxy-containing acrylic resin (a) and polycarboxylic acid compoundcuring agent (b). Likewise, the thermosetting powder coating compositionaccording to the first aspect of the present invention may also containthe resin (d) that exists in the liquid form at room temperature,besides the above-described resin (a) and curing agent (b), for thepurposes of improving smoothness of resulting films. The “liquid form”,as used herein, means the state wherein a substance has a definitevolume but does not have a definite configuration. The “liquid form”does not mean the state of a resin solution formed by dissolving a resinin a solvent such as an organic solvent, but means the liquid state of aresin under the substantial absence of a solvent. In the below-describedwet process for use in the production of the thermosetting powdercoating composition of the present invention, a resin may be used in theform of a resin solution. It should be understood, however, that asolvent, such as an organic solvent, is left substantially unremained inthe final thermosetting powder coating composition of the presentinvention.

The type of the resin (d) that exists in the liquid form at roomtemperature is not particularly specified. Examples of such resins (d)include a polyester resin, an acrylic resin and other various compounds.The resin (d) can be prepared by techniques known to those skilled inthe art, such as polycondensation, free-radical polymerization and thelike.

For the increased performances and physical properties of resultingfilms, it is preferred that the resin (d) that exists in the liquid format room temperature has a curable group, examples of which includehydroxyl, carboxyl and epoxy groups. The curable group content is notparticularly specified, and may be suitably chosen depending upon theavailability in industry and handling properties of resins containingsuch curable groups, and the performances and physical propertiesdesired for the resulting films.

In the case where the resin (d) contains a curable functionality in theform of a hydroxyl group, a hydroxyl value of the resin solids is notparticularly specified. In an exemplary case where the resin (d) is apolyester resin, the hydroxyl value is preferably in the range of30-250, more preferably in the range of 50-200. In another exemplarycase where the resin (d) is an acrylic resin, the hydroxyl value ispreferably up to 100, more preferably up to 50. The resin (d) may alsocomprise a polyol compound. In such a case, a hydroxyl value of theresin solid is preferably in the range of 30-300, more preferably in therange of 50-250. Examples of commercial polyol compounds include PLAXEL410C, PLAXEL 308 and PGL06 (respectively manufactured by Daicel Chem.Ind. Co., Ltd.).

In the case where the resin (d) contains a curable functionality in theform of a carboxyl group, a carboxyl-associated acid value of the resinsolids is not particularly specified. In an exemplary case where theresin (d) is a polyester resin, such an acid value is preferably in therange of 1-180, more preferably in the range of 5-80. In anotherexemplary case where the resin (d) is an acrylic resin, the acid valueis preferably up to 100, more preferably up to 50.

In the case where the resin (d) contains a curable functionality in theform of an epoxy group, an epoxy value of the resin solid is notparticularly specified. In an exemplary case where the resin (d) is anacrylic resin, the epoxy value is preferably up to 180, more preferablyup to 120. The resin (d) may comprise a polyepoxy compound. In such acase, an epoxy value of its solids component is preferably in the rangeof 20-650, more preferably in the range of 30-200. Examples of polyepoxycompounds include ST1000, ST3000, YH-300, DINACOL EX-301, DINACOLEX-411, DINACOL EX-701, DINACOL EX-212 (respectively manufactured byTohto Kasei Co., Ltd.); PUE-106, PUE-107, CELLOXIDE 2021 (respectivelymanufactured by Daicel Chem. Ind. Co., Ltd.); glycidyl esters ofhexahydrophthalic anhydride and the like.

Preferably, the resin (d) that exists in the liquid form at roomtemperature comprises the aforementioned polyester or acrylic resin thatexists in the liquid form at room temperature, in considerations ofavailability in industry and design freedom.

A number average molecular weight of the resin (d) present in the liquidform at room temperature is not particularly specified, but ispreferably in the range of 500-5,000 for the improved blockingresistance and smoothness of resulting films. While not particularlyspecified, Tg of the resin (d) is preferably up to 25° C. for theimproved blocking resistance, performances and physical properties ofresulting films.

While not particularly specified, an SP value of the resin (d) thatexists in the liquid form at room temperature is generally in the rangeof 9.0-12.0, preferably in the range of 9.0-11.0, more preferably in therange of 9.5-11.0.

In the case where the epoxy-containing acrylic resin (a) in thethermosetting powder coating composition of the present inventioncontains the aforementioned epoxy-containing acrylic resin A andepoxy-containing acrylic resin B, the resin (d) that exists in theliquid form at room temperature preferably satisfies the followingconditions:

(1) (SP_(B)-SP_(d)) is within the range of 0.01-1.5, where SP_(d) is asolubility parameter of resin (d); and

(2) The solids component of resin (d) is 5-70 parts by weight, based on100 parts by weight of the total solids of the aforementionedepoxy-containing acrylic resin (a) and resin (d).

If (SP_(B)-SP_(d))<0.01, the blocking resistance of the resulting powdercoatings may deteriorate. If (SP_(B)-SP_(d))>1.5, the resulting powdercoatings, when formed into films, may provide the poor appearance.

Also, if the solids weight of the resin (d) is below 5 parts, based on100 parts of the total solids weight of the aforementionedepoxy-containing acrylic resin (a) and resin (d), the improvement insmoothness of resulting films may become insufficient. If it exceeds 70parts, the blocking resistance may decrease. Furthermore, the solidsweight of resin (d) is preferably 5-50 parts, more preferably 5-20parts, based on 100 parts of the total solids weight of theaforementioned epoxy-containing acrylic resin (a) and resin (d).

A molar ratio of the carboxyl to epoxy content of the thermosettingpowder coating composition of the present invention is preferably in therange of 5/10-11/10, more preferably in the range of 7/10-10/10. If themolar ratio falls outside the above-specified range, the thermosettingpowder coating composition may result in the insufficiently hardenedfilms.

Optional Components (e)

The thermosetting powder coating composition of the present inventionmay further contain a coloring component. In the powder coating, powderparticles left uncoated on a substrate are generally recovered forreuse. The reuse of such a recovered powder, however, leads often tocolor mixing. It is generally preferred that the thermosetting powdercoating composition is of clear type. This so-called thermosetting clearpowder coating composition contains no coloring component or may containa coloring component to the extent that does not adversely affect thetransparency of the composition. The coloring component may comprise acoloring pigment, dye or the like, for example.

Examples of coloring components include coloring pigments such astitanium dioxide, red iron oxide, yellow iron oxide, carbon black,phthalocyanine pigments, quinacridone pigments and azo pigments; dyesand the like.

Components other than the above-described components, e.g., variousadditives generally used in the powder coating art can be optionallyadded to the thermosetting powder coating composition of the presentinvention.

Such additives include, for example, extender pigments such as talc,silica, calcium carbonate and precipitated barium sulfate; fluidityenhancers such as AEROSIL 130 and AEROSIL 200 (manufactured by JapanAerosil Co., Ltd.); surface control agents including silicones such asdimethyl silicone and methyl silicone, and acrylic oligomers;anti-foaming agents represented by benzoins such as benzoin and benzoinderivatives; curing promoters (or curing catalysts); plasticizers;antistatic agents; UV absorbers; antioxidants; pigment dispersingagents; flame retardants; fluidity enhancers; and curing promoters (orcuring catalysts) such as amine compounds, imidazole compounds andcationic polymerization catalysts and the like.

Although not particularly specified, a volume mean particle diameter ofthe thermosetting powder coating composition of the present invention isgenerally in the range of 5-40 μm, preferably in the range of 5-30 μm,more preferably in the range of 5-20 μm from the viewpoint of smoothnessand appearance of resulting films.

A wet process suitable for use in the preparation of the thermosettingpowder coating composition of the present invention include thefollowing steps:

1. A raw material solution is prepared by allowing the above-describedraw material to dissolve or disperse in an organic solvent.

2. The raw material solution obtained in step 1 is added to an aqueoussolution containing a water-soluble polymer that has a clud point in the30-90° C. temperature range and mix them at a temperature of below thecloud point to thereby produce a suspension.

3. The suspension obtained in step 2 was heated to a temperature equalto or higher than the cloud point to form secondary oil particles andthe organic solvent is distilled off from a system to thereby collectparticles. For the purposes of disclosure of the present invention, thistype of process shall hereinafter be referred to as a cloud point wetprocess.

Step 1: Raw Material Solution Preparation

In a first step of the wet process, the above-described raw material isallowed to dissolve or disperse in an organic solvent to thereby preparea raw material solution. This step is performed regardless of whetherthe utilization of cloud point of the water-soluble polymer is presentor absent. The curing agent composition, which is one component of theraw material, must be maintained in a dispersed form in the raw materialsolution prepared in this first step. If the curing agent composition isallowed to dissolve completely into the raw material solution, thesubstantial absence of the curing agent composition results. This leadsto the failure to obtain a purposed effect. Preferably, the curing agentcomposition is finely dispersed to the extent that it does not remain asdust-size particles in the below-described filtering operation.

The epoxy-containing acrylic resin (a) for use as one component of theraw material can be chosen from those described above.

The polycarboxylic acid compound curing agent (b) for use as onecomponent of the raw material can also be chosen from those describedabove. The polycarboxylic acid compound curing agent (b) may comprise amixture prepared by mixing the polycarboxylic acid compound (b-1) thatexists in the crystalline solid form at room temperature with thecarboxylic acid compound (b-2). Preferably, the mixing thereof isachieved sufficiently for the better performances and physicalproperties of resulting films.

As stated earlier, two techniques can be used to mix those twocompounds. One technique involves mixing the two compounds afterparticle size reduction thereof. Another technique involves mixing themin the liquid form. The latter technique is preferred for its ability toprovide a more uniform mixture leading to the improved smoothness ofresulting films.

The polycarboxylic acid compound (b-1) and carboxylic acid compound(b-2) may be mixed such that a molar ratio of a carboxyl content of thepolycarboxylic acid compound curing agent (b) to an epoxy content of theepoxy-containing acrylic resin (a) falls with the range of 5/10-11/10,preferably within the range of 7/10-10/10.

When needed, other components such as pigments, a variety of additivesand the like may be added to the raw material.

The useful organic solvent may show a substantial water immiscibility,i.e., a water-solubility of not exceeding 10%, and have a boiling pointof below 100° C. at normal pressure. Alternatively, it may have atendency to form an azeotropic mixture with water. Specific examples ofuseful organic solvents include xylene, toluene, cyclohexane, ethylacetate and the like.

A solids weight of the above-described raw material in the raw materialsolution is not particularly specified. For example, it may be adjustedto fall within the range of 10-90% by weight, preferably in the range of30-90% by weight. In the case where the epoxy-containing acrylic resin(a) and/or fine crosslinked resin particles are provided in the form ofa liquid dissolved in an organic solvent, an additional supply of anorganic solvent is not required if the solids weight of the raw materialsolution is already within the target range.

When necessary, such organic solvent insolubles as those initiallyincorporated in the raw material, e.g., dust or foreign matterintroduced while the raw material solution is prepared, may be removedby filtration. The filtration can be achieved according to conventionalprocedures known to those skilled in the art, as by using various typesof filters.

Step 2: Suspension Preparation at a Temperature Below a Cloud Point

In a second step of the wet process, the raw material solution obtainedin step 1 is added to an aqueous solution containing a water-solublepolymer that has a cloud point in the 30-90° C. temperature range andmix them at a temperature of below the cloud point to thereby prepare asuspension containing primary oil particles.

The water-soluble polymer that has a cloud point in the 30-90° C.temperature range is not particularly specified. Illustrative of suchwater-soluble polymers are those which, when heated in the form ofaqueous solutions, show a cloud point phenomenon in the 30-90° C.temperature range, examples of which include partially saponifiedpolyvinyl alcohol having a saponification value of below 85%; partiallyformalized compounds; polyvinyl alcohol based polymers containinglocalized hydrophobic groups such as ethylene-vinyl alcohol copolymer;cellulose derivatives such as methyl cellulose and hydroxypropylcellulose; polyethylene glycol alkyl ethers; a block copolymer ofethylene glycol and propylene glycol and the like. Besides,water-soluble polymers that do not have inherent cloud points can bemade usable by adding thereto suitable electrolytes so that a cloudpoint phenomenon occurs in the 30-90° C. temperature range. Thewater-soluble polymers listed above as having cloud points can be usedalone or in any combination thereof.

In technical considerations such as particle size control, the use ofthe aforementioned water-soluble polymer that do not have cloud pointsis preferred.

The type of the water-soluble polymer that does not have a definitecloud point is not particularly specified. Illustrative of thewater-soluble polymers that do not have cloud points are those which,even if heated in the form of their aqueous solutions, do not give riseto a cloud point phenomenon at 100° C. or below, specific examples ofwhich include completely saponified polyvinyl alcohol, partiallysaponified polyvinyl alcohol having a saponification value of not below85%, ethyl cellulose, hydroxyethyl cellulose, polyethylene glycol andthe like. The above-listed water-soluble polymers that do not have cloudpoints may be used alone or in any combination thereof. In the casewhere such a water-soluble polymer that does not have a cloud point isused in combination with the aforementioned water-soluble polymer thathas a cloud point in the 30-90° C. temperature range, a ratio in solidsweight of the water-soluble polymer that does not have a cloud point tothe water-soluble polymer that has a cloud point in the 30-90° C.temperature range is preferably from 99/1 to 10/90. It may be difficultto control particle size of the secondary oil particles which will bedescribed later, when the ratio is the outside of this range.

In this step, the above-described water-soluble polymer that has a cloudpoint in the 30-90° C. temperature range is first charged, in the formof its aqueous solution, into a reactor equipped with a stirrer.Preferably, the aqueous solution contains the water-soluble polymer inconcentration of 0.02-20% by weight. This concentration range betterassures the homogeneity of the solution.

The raw material solution obtained in step 1 is then added to theaqueous solution of water-soluble polymer, followed by mixing thereof ata temperature of below the cloud point of the water-soluble polymer. Inthe case where two or more of water-soluble polymers that have cloudpoints in the 30-90° C. temperature range are used in combination, thelowest cloud point becomes a controlling parameter. The temperature inthe following steps shall be specified by the lowest cloud point of thewater-soluble polymers used.

In step 1, each component material may be rendered into its solution. Insuch a case, individual component material solutions may be separatelyadded to the aqueous solution of water-soluble polymer in step 2. It ishowever preferred that all the individual component material solutionsare mixed together to provide the raw material solution before additionto the aqueous solution of water-soluble polymer. This results inobtaining primary oil particles which contain more uniformly distributedcomponent materials.

The raw material solution is added to the aqueous solution ofwater-soluble polymer such that a ratio of the weight of the aqueouswater-soluble polymer to the solids weight of the raw material solutionis preferably maintained within the range of 0.5/1-3/1. This provides amore uniform liquid mixture. The liquid mixture is stirred and dilutedwith an ion exchange water, if necessary, to finally obtain a suspensioncontaining primary oil particles, with the solids weight of the rawmaterial being in the range of 10-50%. The stirring can be achieved by astirrer chosen from those known to those skilled in the art dependingupon the viscosity of the liquid mixture.

The primary oil particles preferably have a volume mean particlediameter of not exceeding 15 μm, more preferably of not exceeding 10 μm.The mean particle diameter of the primary oil particles can bedetermined by measuring particle diameters of arbitrarily sampledprimary oil particles.

As analogously to step 1, organic solvent insolubles, such as dust orforeign matter introduced either prior to or during step 2, may beremoved by filtration. Preferably, the filtration is performed at leastonce before the primary oil particles are solidified in the followingstep.

Preferably, the organic solvent is partially distilled off in step 2.This serves to improve properties of the resulting thermosetting powdercoating composition. The organic solvent can be distilled off if thesuspension is maintained at a constant temperature below the cloud pointas described above. However, the organic solvent can be distilled offmore efficiently if the heat applied to form secondary oil particles inthe following step is utilized. In view of the thermosetting nature ofthe raw material solution, the organic solvent is preferably distilledoff at a lower temperature at reduced pressure. In the case where theorganic solvent is distilled off by maintaining the suspension at aconstant temperature below the cloud point, such a procedure may becarried out so that the amount of the organic solvent remaining in theprimary oil particles is reduced to 30% by weight or below, preferably10% by weight or below, more preferably 5% by weight or below.

Step 3: Formation of Secondary Oil Particles and Particle Collection

In a third step of the wet process, the suspension obtained in step 2 isheated to a temperature equal to or higher than the cloud point tothereby form secondary oil particles and the organic solvent isdistilled off to collect particles.

First, the suspension obtained in step 2 is heated to a temperatureequal to or higher than the cloud point so that the primary oilparticles are caused to agglomerate to form secondary oil particles. Thedegree of agglomeration is monitored by periodically sampling secondaryoil particles for measurement of their particle. sizes. At the pointwhen their particle sizes come to fall within a targeted range, heatingmay be terminated. The heating conditions may be suitably chosendepending upon the type of the water-soluble polymer used and the natureof the raw material solution.

Two techniques can be utilized to control the particle sizes ofsecondary oil particles within the targeted range. One techniqueinvolves adjusting a weight ratio of the water-soluble polymer that doesnot have a cloud point to the water-soluble polymer that has a cloudpoint in the 30-90° C. temperature range. Another technique involvesreducing a temperature of the suspension to below the cloud point of thewater-soluble polymer when the particle sizes of the secondary oilparticles have reached the target range to thereby terminate the furthergrowth of secondary oil particles via agglomeration.

The water-soluble polymer that has a cloud point in the 30-90° C.temperature range is believed to participate in the formation ofsecondary oil particles as a result of agglomeration of primary oilparticles. When the water-soluble polymer that does not have a cloudpoint is also used in combination therewith, this water-soluble polymerthat does not have a cloud point is believed to participate in theparticle size control of the primary oil particles.

The removal of the organic solvent results in the solidification of thesecondary oil particles. Such a desolvating operation is preferablycontinued until the complete solidification of the primary oil particlesis ascertained.

The organic solvent can be distilled off by the application of heatand/or reduced pressure. In view of the thermosetting nature ofparticles obtained as a result of solidification, the organic solvent ispreferably distilled off at a lower temperature by reducing pressure inthe system. Such a desolvating operation is preferably continued untilthe complete solidification of primary oil particles is ascertained.

The particles obtained as a result of solidification of primary oilparticles can be isolated by conventional solid-liquid separationtechniques such as filtration and centrifugation.

Such particles, after rinsed with water and dried, finally result in apowder coating composition.

The powder coating composition thus obtained has a volume mean particlediameter in the range of 5-40 μm, preferably in the range of 5-30 μm,more preferably in the range of 5-20 μm.

Other than the cloud point wet process, the wet process which does notutilize the cloud point (may be hereinafter referred to as anon-clouding wet process) can be employed to obtain the thermosettingpowder coating composition of the present invention. The thermosettingpowder coating composition having the better controlled particle sizeand the sharper particle size distibution is obtained by utilizing thecloud point wet process relative to utilizing the non-clouding wetprocess.

Vaious methods known to those skilled in the art, such as an electricresistance method and laser light scattering method, can be utilized todetermine a volume mean particle diameter and a number mean particlediameter in the present invention.

In the non-clouding wet process for producing the thermosetting powdercoating composition of the present invention, the water-solvent polymer,regardless of whether or not it has a cloud point in the 30-90° C.temperature range, is used in step 1. In the case where the non-cloudingwater-soluble polymer alone is used, a temperature used in step 2 is notparticularly specified. In the other case where the water-solublepolymer which has a cloud point in the 30-90° C. temperature range ispresent, stirring must be supplied at a temperature of below the cloudpoint. Furthermore, a third step is omitted and thus no secondary oilparticle is formed.

A fluidity enhancer, such as AEROSIL 130 or AEROSIL 200 (manufactured byJapan Aerosil Co., Ltd.) may further be externally added onto particlesurfaces of the thermosetting powder coating composition. The use of thefluidity enhancer not only imparts the improved fluidity to the powdercoating composition but also improves the blocking resistance. Such afluidity enhancer can be deposited on surfaces of powder particles bymixing it with the thermosetting powder coating composition.

The thermosetting powder coating composition of the present inventionmay be applied onto a substrate, as by an electrostatic coatingtechnique, generally to a thickness of 40-80 μm. The applied compositionis then heated to form a coating film. Examples of substrates includesurface treated or untreated plastics, iron plate, steel plate, aluminumplate and the like. The heating temperature can be suitably chosendepending upon the particular type of the thermosetting powder coatingcomposition used, but is generally in the range of 100-200° C. Theheating period may be suitably adjusted depending upon the heatingtemperature selected.

Multilayer Film-Forming Method

The multilayer film-forming method of the present invention includes thesteps of applying a basecoat onto an undercoated and optionallyintercoated substrate, applying the powder coating composition of thepresent invention onto the basecoat, and heating the substrate carryingthereon the basecoat and powder coating layers.

The substrate for use in the multilayer film-forming method of thepresent invention is previously undercoated and optionally intercoated.Examples of suitable substrates are listed in the earlier descriptionsof the film-forming method. Known coatings, such as primers,electrodeposition coatings and the like, can be used to form theundercoating or intercoating layer.

The type of the basecoat is not particularly specified, and may be asolvent-or water-borne. In considerations of environmental protection,the use of the water-borne base coat is preferred. This base coat isapplied onto the undercoated and optionally intercoated substrate, as byan electrostatic spray equipment, to a thickness of 10-20 μm.

The substrate thus coated with the base coat is preheated at 60-100° C.for about 5-10 minutes, as by exposure to infrared ray or hot air.Subsequently, the thermosetting powder coating composition of thepresent invention is applied onto the base coat, as by an electrostaticspray coating technique, to a thickness of 40-80 μm. Heating is thenapplied at a temperature as appropriate to the thermosetting powdercoating composition used. The heating temperature is generally in therange of 100-200° C., preferably in the range of 120-180° C., morepreferably in the range of 130-160° C. The heating period can besuitably adjusted depending upon the heating temperature selected, butmay be preferably in the range of 5-40 minutes, more preferably in therange of 10-25 minutes.

The practice of the multilayer film-forming method of the presentinvention thus results in the formation of a multilayer coating film.

DESCRIPTION OF PREFERRED EXAMPLES

The following Examples illustrate the first aspect of the presentinvention

Preparation Example 1

Preparation of an Emulsifier Containing Amphoionic Groups

A reaction vessel equipped with a stirrer, thermo-regulator, reflux tubeand decanter was charged with 134 parts by weight of bis(hydroxyethyl)taurine, 130 parts by weight of neopentyl glycol, 236 parts by weight ofazelaic acid, 186 parts by weight of phthalic anhydride and 27 parts byweight of xylene. The mixture was then heated under a nitrogenatmosphere and the water produced during the reaction was distilled offas an azeotrope with xylene. After the reflux of xylene was initiated,the temperature of the mixture was gradually raised to 190° C. about 2hours during which period the reaction was continued so that an acidvalue of resin solids reached 145. The mixture was then cooled to 140°C. to provide a resin solution.

Then, 314 parts by weight of “CARDURA E10” (monoepoxide manufactured byShell Chem., Co., Ltd.) was added dropwise to the resin solution over 30minutes. After successive two hours of stirring, the reaction wasterminated to obtain an emulsifier. The emulsifier was found to have aresin solids acid value of 59, a resin solids hydroxyl value of 90 and anumber average molecular weight of 1054.

Preparation Example 2

Preparation of an Emulsion 1

Into a reaction vessel equipped with a stirrer, condenser andthermoregulator, 232 parts by weight of deionized water, 15 parts byweigh of the emulsifier obtained in Preparation Example 1 and 0.75 partsby weight of dimethyl ethanolamine were charged and then maintained at80° C. with stirring to provide a liquid mixture. A solution prepared bydissolving 4.5 parts by weight of azobiscyanovaleric acid into 45 partsby weight of deionized water and 4.3 parts by weight of dimethylethanolamine was added to the liquid mixture to prepare an emulsion 1.

Preparation Example 3

Preparation of a Dispersion Containing Fine Crosslinked Resin ParticlesM-1 and of a Crosslinked Resin Powder M-1′

A liquid mixture of 50 parts by weight of tert-butyl methacrylate and 50parts by weight of ethylene glycol dimethacrylate was added dropwise tothe emulsion 1 obtained in Preparation Example 2 over 1 hour.Thereafter, a solution formed by dissolving 1.5 parts by weight ofazobiscyanovaleric acid into 15 parts by weight of deionized water and1.4 parts by weight of dimethyl ethanolamine was added to the liquidmixture which was subsequently stirred at 80° C. for an additional onehour to prepare an emulsion. This emulsion was then azeotropicallydistilled to replace the ionized water, as a solvent, with xylene,thereby obtaining a dispersion containing fine crosslinked resinparticles M-1. The dispersion of fine crosslinked resin particles M-1was measured for a volume mean particle diameter and a solids content.The results are given in Table 1. The volume mean particle diameter wasdetermined by using a coulter counter (manufactured by Beckman CoulterCo., Ltd.).

A fraction of the dispersion containing fine crosslinked resin particlesM-1 was collected and heated under a reduced pressure to distill xyleneoff. This resulted in obtaining a crosslinked resin powder M-1′ having asolids content by weight of 100%.

Preparation Examples 4-7

Preparation of Dispersions Containing Fine Crosslinked Resin ParticlesM-2 through M-5

The procedure of Preparation Example 3 was followed, except that theformulations shown in Table 1 were used, to prepare four differentdispersions respectively containing fine crosslinked resin particles M-2through M-5. Each dispersion was measured for a mean volume particlediameter and a solids content in the same manner as in PreparationExample 3. The collected data are given in Table 1.

TABLE 1 Preparation Examples Fine Crosslinked Resin 3 4 5 6 7 ParticlesM1 M2 M3 M4 M5 Components Styrene — 50 40 30 40 Methyl Methacrylate — —10 20 — n-Butyl Acrylate — — — — 20 tert-Butyl 50 — — — — MethacrylateEthylene Glycol 50 45 50 50 20 Dimethacrylate Glycol Dimethacrylate — 5— — 20 Azobiscyanovaleric 1.5 1.5 1.5 1.5 1.5 Acid Deionized Water 15 1515 15 15 Dimethyl Ethanol- 1.4 1.4 1.4 1.4 1.4 amine Measured VolumeMean Particle 53 52 49 70 64 Values Diameter (nm) Solids Weight (%) 2020 20 20 20

Preparation Example 8

Preparation of an Emulsion 2

Into a reaction vessel equipped with a stirrer, condenser andthermoregulator, 232 parts by weight of deionized water, 10 parts byweight of the emulsifier obtained in Preparation Example 1 and 0.75parts by weight of dimethyl ethanolamine were charged and thenmaintained at 80° C. with stirring to provide a liquid mixture. Asolution formed by dissolving 0.2 parts by weight of azobiscyanovalericacid into 20 parts by weight of deionized water and 0.26 parts by weightof dimethyl ethanolamine was added to the liquid mixture to prepare anemulsion 2.

Preparation Example 9

Preparation of a Dispersion Containing Fine Crosslinked Resin ParticlesM-6

A first charge of resin materials, i.e., a liquid mixture of 15 parts byweight of styrene, 45 parts by weight of n-butyl acrylate and 40 partsby weight of ethylene glycol dimethacrylate was added dropwise to theemulsion 2 obtained in Preparation Example 2 over one hour, and thenmaintained at 80° C. with stirring for an additional one hour. A liquidmixture containing 0.8 parts by weight of azobiscyanovaleric aciddissolved in 25 parts by weight of deionized water and 0.3 parts byweight of dimethyl ethanolamine was added. A second charge of resinmaterials, i.e., a liquid mixture of 15 parts by weight of styrene, 45parts by weight of n-butyl acrylate and 40 parts by weight of ethyleneglycol dimethacrylate was further added dropwise over one hour.Thereafter, a liquid mixture containing 1.5 parts by weight ofazobiscyanovaleric acid dissolved in 15 parts by weight of deionizedwater and 1.4 parts by weight of dimethyl ethanolamine was further addedand then maintained at 80° C. with stirring for another one hour toprovide an emulsion. This emulsion was then treated in the same manneras in Preparation Example 3 to replace a solvent from the ionized waterto xylene, thereby obtaining a dispersion containing fine crosslinkedresin particles M-6. The dispersion of fine crosslinked resin particlesM-6 was measured for a volume mean particle diameter and a solidscontent in the same manner as in Preparation Example 3. The results aregiven in Table 2.

Preparation Example 10

Preparation of a Dispersion Containing Fine Crosslinked Resin ParticlesM-7

The procedure of Preparation Example 9 was followed, except that theformulation given in Table 2 was used, to prepare a dispersioncontaining fine crosslinked resin particles M-7. The dispersioncontaining fine crosslinked resin particles M-7 was measured for avolume mean particle diameter and a solids content in the same manner asin Preparation Example 3. The data collected are shown in Table 2.

TABLE 2 Preparation Examples Fine Crosslinked Resin 9 10 Particles M6 M71st Stage Resin Formulations Styrene 15 — 2-Ethylhexyl Acrylate — 40n-Butyl Acrylate 45 — tert-Butyl Methacrylate — — Ethylene Glycol 40 60Dimethacrylate Azobiscyanovaleric Acid 0.8 0.5 Deionized Water 25 25Dimethyl Ethanolamine 0.3 0.3 2nd Stage Resin Formulations Styrene 15 60n-Butyl Acrylate 45 — Ethylene Glycol 40 40 DimethacrylateAzobiscyanovaleric Acid 0.8 0.8 Deionized Water 15 15 DimethylEthanolamine 1.4 1.4 Measured Values Volume Mean Particle 77 74 Diameter(nm) Solids Weight (%) 20 20

Preparation Example 11

Preparation of a Dispersion Containing a Curing Agent H-1

1,10-decane dicarboxylic acid was dispersed in xylene, and ground by asand grinding mill to obtain a dispersion containing a curing agent H-1(solids content by weight of 30%). The volume mean particle diameter ofthe resulting dispersion was 6 μm when measured by a coulter counter(manufactured by Japan Coulter Co., Ltd.).

Preparation Example 12

Preparation of a Dispersion Containing a Curing Agent Composition H-2

A mixture containing, by weight, 75 parts of 1,10-decane dicarboxylicacid and 25 parts of sebacic acid was dispersed in xylene, and ground bya sand grinding mill to obtain a dispersion containing a curing agentcomposition H-2 (solids content by weight of 30%). The volume meanparticle diameter of the resulting dispersion was measured in the samemanner as in Preparation Example 11, which gave the value of 6 μm. Also,a fraction of the resulting dispersion was collected and placed under areduced pressure to remove xylene therefrom. The melting point of theresultant substance was determined by DSC 220C (differential scanningcalorimeter manufactured by Seiko Instrument Ind. Co., Ltd., heatingrate of 5° C./min.) which gave the result of 120° C.

Preparation Example 13

Preparation of a Solution Containing an Epoxy-Containing Acrylic ResinR-1

A reaction vessel equipped with a stirrer, thermoregulator and refluxtube was charged with 63 parts by weight of xylene which wassubsequently placed under nitrogen atmosphere and heated to atemperature of 130° C. A mixture containing the below-specifiedcomponents was added dropwise over three hours:

glycidyl methacrylate: 45 parts by weight styrene: 20 parts by weightn-butyl methacrylate 20 parts by weight isobutyl methacrylate: 10 partsby weight n-butyl acrylate: 5 parts by weight tert-butyl peroctoate: 3parts by weight.

After completion of the dropwise addition, the mixture was maintained at130° C. for additional 3 hours and then cooled to a room temperature tothereby obtain a solution containing an epoxy-containing acrylic resinR-1 (epoxy equivalents of resin solids=180, and solids concentration=60weight %). Also, a fraction of the resin R-1 solution was heated under areduced pressure to distill xylene off to provide the resin R-1. Tg ofthe resulting resin R-1 was determined by DSC 220C (manufactured bySeiko Instrument Ind. Co., Ltd., heating rate of 5° C./min.) which gavethe result of 30° C. The measurement according to a turbidimetry methodrevealed an SP value of 10.2. GPC (gel permeation chromatography)revealed a number average molecular weight of 9,000.

Preparation Examples 14-16

Preparation of Epoxy-Containing Acrylic Resins R-2 through R-4

The procedure of the above Preparation Example 13 was followed, exceptthat the formulations shown in Table 3 were used, to obtainepoxy-containing acrylic resins R-2, R-3 and R-4. These resins obtainedwere respectively measured for Tg, SP value and number average molecularweight in the same manner as in Preparation Example 13. The measured Tg,SP value and molecular weight for each resin are given in Table 3.

TABLE 3 Preparation Examples 13 14 15 16 Resin R1 R2 R3 R4 ComponentsStyrene 20 20 20 20 Methyl — 27 — 20 Methacrylate Glycidyl 45 45 60 60Methacrylate Isobutyl 10 5 — — Methacrylate 2-Hydroxyethyl — 3 — —Methacrylate n-Butyl 20 — 15 — Methacrylate n-Butyl Acrylate 5 — 5 —t-Butyl Peroctoate 3 10 5 7 Character- Resin Solids Epoxy 180 180 240240 istic Equivalent (g/eq) Values Tg (° C.) 30 70 40 70 SP Value 10.210.6 10.5 10.9 Number Average 9000 3000 5000 3500 Molecular Weight

EXAMPLE 1

Thermosetting Powder Coating Composition C-1

The components specified below were mixed in a sand grinding mill toprepare a raw material solution.

Components Parts by Weight epoxy-containing acrylic resin R-1 103.3solution (solids content of 60 wt. %) epoxy-containing acrylic resin R-225.8 solution (solids content of 60 wt. %) curing agent H-1 dispersion(solids content 75.0 of 30 wt. %) dispersion of fine crosslinked resin6.0 particles M-1 (solids content of 20 wt. %) silicone-based surfaceconditioning agent 0.2 acrylic-based surface conditioning agent 0.1benzoin 0.5 UV absorber 1.0 anti-oxidant 1.0

The raw material solution was then added to an aqueous polymercontaining, by weight, 6 parts of GOSENOL GH-20 (polyvinyl alcoholmanufactured by Nippon Gosei Kagaku Co., Ltd., saponification level of88%, no cloud point), 3 parts of GOSENOL KL-05 (polyvinyl alcoholmanufactured by Nippon Gosei Kagaku Co., Ltd., saponification level of80%, cloud point of about 80° C.), 1 part of hydroxypropyl cellulose(cloud point of about 50° C.) and 90 parts of deionized water. Aresulting mixture was further mixed at 25° C. by using a homogenizer toprepare a suspension containing primary oil particles. These primary oilparticles present in the suspension were measured for a volume meanparticle diameter in the same manner as in Preparation Example 3. Theresults are given in Table 4.

The suspension was diluted by the addition of 300 parts by weight ofdeionized water, and then transferred to a reaction vessel equipped witha stirrer, thermoregulator, reflux tube and vacuum apparatus.

The suspension was reduced in pressure to 30 Torr. and then heated to35° C. After a time interval, the suspension was further reduced inpressure to 140 Torr. and then heated to 60° C. to obtain secondary oilparticles. Subsequently, a solvent present in a dispersed phase wasdistilled off completely to solidify the primary oil particles intoparticles. The resulting suspension was cooled and filtered undersuction to collect particles which were subsequently dried at 30° C.under a reduced pressure to obtain a thermosetting powder coatingcomposition C-1. This powder coating composition was measured for avolume mean particle diameter and a number mean particle diameter, asanalogously to Preparation Example 9. The measurement results, as wellas a ratio of the volume mean to number mean particle diameter, aregiven in Table 4.

EXAMPLES 2-11

Preparation of Thermosetting Powder Coating Compositions C-2 throughC-11

The procedure of Example 1 was followed, except that the formulationsshown in Table 4 were used, to obtain thermosetting powder coatingcompositions C-2 through C-11. In each Example, a volume mean particlediameter of primary oil particles, volume mean and number mean particlediameters of the resulting powder coating composition were measured inthe same manner as in Example 1. A ratio of the volume mean to numbermean particle diameter was also calculated. The results are given inTable 4.

Comparative Examples 1 and 2

Thermosetting Powder Coating Compositions C-12 and C-13 Prepared in theAbsence of Fine Crosslinked Resin Particles

The procedure of Example 1 was followed, except that the formulationsexcluding fine crosslinked resin particles M-1 dispersion as shown inTable 4 were used, to obtain thermosetting powder coating compositionsC-12 and C-13. In each Comparative Example, a volume mean particlediameter of primary oil particles, volume mean and number mean particlediameters of the resulting powder coating composition were measured inthe same manner as in Example 1. A ratio of the volume mean to numbermean particle diameter was also calculated. The results are given inTable 4.

Comparative Example 3

Thermosetting Powder Coating Composition C-14 Prepared with ExternalAddition of Fine Crosslinked Resin Particles

The procedure of Example 1 was followed, except that the dispersion ofcrosslinked resin particles M-1 was not used, to obtain a thermosettingpowder coating composition. The resulting coating composition and thefine crosslinked resin particles M-1′ obtained in Preparation Example 1,based on the formulation shown in Table 4, were mixed by a Henschelmixer to prepare a thermosetting powder coating composition C-14. Avolume mean particle diameter of primary oil particles, volume mean andnumber mean particle diameters of the resulting powder coatingcomposition were measured in the same manner as in Example 1. A ratio ofthe volume mean to number mean particle diameter was also calculated.The results are given in Table 4.

Evaluation Tests

The thermosetting powder coating compositions obtained in Examples 1-11and Comparative Examples 1-3 were evaluated for the followingproperties.

A. Smoothness

SUPERLAC M260-SILVER (water-borne metallic basecoat manufactured byNippon Paint Co., Ltd.) was applied onto intercoated substrates by anelectrostatic coating technique to a thickness of 10-20 μm, andpreheated in a hot-air oven at 145° C. for 25 minutes. After thesubstrates were cooled to room temperature, each thermosetting powdercoating composition was electrostatically applied, in the form of alayer having a thickness of 50 μm, to a set of two substrates. One wasset parallel to horizontal and the other at an angle of 60 degrees fromhorizontal within a hot-air oven and the layers applied thereon werebaked at 145° C. for 25 minutes to form coating films. This resulted inobtaining two sample pieces carrying the coating films respectivelyformed in horizontal and vertical orientations. The resulting samplepieces were taken out from the oven and cooled to room temperature. Thecoating films respectively formed on horizontally and verticallyoriented substrates were evaluated in terms of an NSIC value (%) and anNSIC* value (%) as respectively measured by an image sharpness measuringapparatus (manufactured by Suga testing machine Co., Ltd.). The coatingfilms having an NSIC value of at least 60% and an NSIC* value of atleast 35% were rated as being satisfactory.

The aforementioned intercoated substrate was prepared byelectrodepositing POWERTOP U-50 (automobile-use cationicelectrodeposition coating manufactured by Nippon Paint Co., Ltd.) onto a100 mm×300 mm×0.8 mm zinc phosphated dull steel plate to a dry thicknessof about 25 μm, baking the deposited layer at 160° C. for 30 minutes,electrostatically depositing ORGA P-2 (intercoat compositionmanufactured by Nippon Paint Co., Ltd.) onto the undercoat to a drythickness of about 40 μm, and baking the deposited layer at 140° C. for30 minutes.

B. Occurrence of Sagging

The coating films formed on vertically oriented substrates, as obtainedabove in evaluating smoothness, were visually observed to evaluate thepresence or absence of sagging. The evaluation results are given inTable 4.

TABLE 4 Examples Comparative Examples 1 2 3 4 5 6 7 8 9 10 11 1 2 3Coating C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 Coating Resin ASolution R2 R2 R2 R2 R4 R4 R4 R4 R4 R4 R4 R2 R4 R2 Formulation (Parts byWeight) 25.8 25.8 25.8 25.8 24.3 24.3 24.3 24.3 24.3 24.3 24.3 25.8 24.325.8 Resin B Solution R1 R1 R1 R1 R3 R3 R3 R3 R3 R3 R3 R1 R3 R1 (Partsby Weight) 103.3 103.3 103.3 103.3 97.3 97.3 97.3 97.3 97.3 97.3 97.3103.3 97.3 103.3 Curing Agent H1 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H1Dispersion (Parts by Weight) 75.0 75.0 75.0 75.0 90.0 90.0 90.0 90.090.0 90.0 90.0 75.0 90.0 75.0 Dispersion of Fine M1 M1 M1 M1 M1 M2 M3 M4M5 M6 M7 — — M1′*) Crosslinked Resin Particles (Parts by Weight) 6.0 6.012.0 1.5 6.0 6.0 6.0 6.0 6.0 6.0 6.0 1.2 Silicone Surface 0.2 0.2 0.20.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Conditioning Agent AcrylicSurface 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Conditioning Agent UV Absorber 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.01.0 1.0 1.0 1.0 Anti-Oxidant 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.01.0 1.0 1.0 Benzoin 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.50.5 Primary Oil Volume Mean Particle 4.8 5.2 3.8 4.3 5.4 4.2 4.7 4.9 5.13.8 4.8 5.0 5.0 4.9 Particles Diameter (μm) Powder Volume Mean Particle15 14 18 14 15 16 13 14 13 15 18 17 17 15 Coating Diameter (μm)Composition Number Mean Particle 6.2 6.4 7.3 7.0 7.2 7.3 5.9 6.0 6.2 7.17.0 6.3 5.9 6.2 Diameter (μm) Volume to Number Mean 2.4 2.2 2.5 2.0 2.12.2 2.2 2.3 2.1 2.1 2.6 2.7 2.9 2.4 Particle Diameter AppearanceHorizontal NSIC(%) 63 65 62 69 70 69 65 66 63 65 69 62 71 60 NSIC*(%) 4144 42 46 48 44 46 45 44 44 47 43 49 42 Vertical NSIC(%) 61 64 59 63 6865 64 62 63 61 62 45 49 52 NSIC*(%) 39 42 39 40 47 41 45 45 40 42 45 2832 35 Sagging Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs. Abs.Pres. Pres. Pres. *)100 wt. % Solids Content Powder

As can be clearly seen from Table 4, the thermosetting powder coatingcompositions containing the epoxy-containing acrylic resin (a),polycarboxylic acid compound curing agent (b) and fine crosslinked resinparticles (c) provide coating films having satisfactory levels ofsmoothness both on horizontally and vertically oriented substrates.

The following Examples illustrate the second aspect of the presentinvention.

Preparation Examples 17-22

Preparation of Epoxy-Containing Acrylic Resins R-5 through R-10

A reaction vessel equipped with a stirrer, thermoregulator and refluxtube was charged with 63 parts by weight of xylene which wassubsequently heated to 130° C. In each Example, a mixture containingdifferent monomers in such proportions as indicated in Table 5 was addeddropwise under nitrogen atmosphere over three hours. After the dropwiseaddition, a resulting mixture was maintained at the same temperature forthree hours and then cooled to room temperature. As a result, solutions(solids content of 65 wt. %) were obtained respectively containingepoxy-containing acrylic resins R-5 through R-10. Tg and an epoxyequivalent of resin solids in each resin solution were determined fromthe formulation indicated in Table 5. For each resin obtained, its SPvalue and number average molecular weight were determined by aturbidimetry method and GPC (gel permeation chromatography),respectively. The results are given in Table 5.

Further, each resin solution was desolvated under a reduced pressurecondition to obtain epoxy-containing acrylic resins R-5 through R-10(solids content of 100 wt. %). Each resin was visually observed for itsform at room temperature. The epoxy-containing acrylic resins R-7 andR-8 were found to be present in the liquid form.

TABLE 5 Preparation Examples 17 18 19 20 21 22 Acrylic Resin R5 R6 R7 R8R9 R10 Components Styrene 20 20 20 20 20 25 Methyl Methacrylate — 27 — —8 60 Glycidyl Methacrylate 45 45 30 30 45 — Isobutyl Methacrylate 5 5 —— — — 2-Hydroxyethyl Methacrylate — 3 — — — 15 Isobutyl Methacrylate — —30 50 — — n-Butyl Methacrylate 20 — — — 11 — n-Butyl Acrylate 5 — 20 — —— Ethyl Acrylate — — — — 16 — t-Butyl Peroctoate 3 10 15 12 3 8.5Characteristic Resin Solids Epoxy Equivalent (g/eq) 180 180 130 130 180— Values Tg (° C.) 30 70 −6 −3 40 100 SP Value 10.2 10.6 10.1 10.1 10.410.8 Number Average Molecular Weight 9000 3000 1700 2000 9000 3500

Preparation Example 23

Preparation of a Polyester Resin P-1

The components specified below were charged into a reaction vesselequipped with a stirrer, thermoregulator and reflux tube where they wereraised in temperature to 220° C. while producing water was removed froma system:

hexahydrophthalic anhydride: 267.0 parts by weight ditrimethylolpropane:87.0 parts by weight cardura E-10 (monoepoxide 283.0 parts by weightmanufactured by Shell): dibutyltin oxide: 1.2 parts by weight

A quantity of the reaction mixture was sampled at proper intervals tomeasure its acid value by titration with a potassium hydroxide solution.Immediately after the acid value reached 5, the reaction mixture wascooled and then diluted with xylene to obtain a polyester resin P-1solution (solids content of 65 wt. %). The resin obtained was found tohave a final acid value of not exceeding 5. Also, its SP value andnumber average molecular weight were determined in the same manner as inPreparation Examples 17-22, which gave the values of 10.1 and 2,000,respectively.

Also, the polyester resin P-1 solution was desolvated under a reducedpressure to obtain a polyester resin P-1 (solids content of 100 wt. %).This resin was visually observed for its form at room temperature andfound to exist in the liquid form.

Preparation Example 24

Preparation of a Polyester Resin P-2

The components specified below were charged into a reaction vesselequipped with a stirrer, thermoregulator and reflux tube where they wereraised in temperature to 220° C. while producing water was removed froma system:

hexahydrophthalic anhydride: 24.0 parts by weight trimethylolpropane:12.0 parts by weight cardura E-10 25.0 parts by weight dibutyltin oxide:0.01 parts by weight

A quantity of the reaction mixture was sampled at proper intervals tomeasure its acid value by titration with a potassium hydroxide solution.Immediately after the acid value reached 5, the reaction mixture wascooled and then diluted with xylene to obtain a polyester resin P-2solution (solids content of 65 wt. %). The resin obtained was found tohave a final acid value of not exceeding 5. Also, its SP value andnumber average molecular weight were determined in the same manner as inPreparation Examples 17-22, which gave the values of 10.0 and 1,500,respectively.

Also, the polyester resin P-2 solution was desolvated under a reducedpressure to obtain a polyester resin P-2 (solids content of 100 wt. %).This resin was visually observed for its form at room temperature andfound to exist in the liquid form.

Preparation Example 25

Preparation of a Dispersion of a Polycarboxylic Acid Compound CuringAgent Composition H-3

Succinic anhydride was dispersed in xylene, and ground by a sandgrinding mill to obtain a dispersion (solids content of 30 wt. %) of apolycarboxylic acid compound curing agent composition H-3. A fraction ofthe resulting dispersion was collected and placed under a reducedpressure to remove xylene therefrom. The measurement of its volume meanparticle diameter by a coulter counter (manufactured by Beckman CoulterCo., Ltd.) gave the result of 6 μm.

Preparation Example 26

Preparation of a Polycarboxylic Acid Compound Curing Agent CompositionH-4 and its Dispersion

Succinic anhydride and 1,10-decanedicaboxylic acid, in proportions insolids weight of 1:3, were mixed and comminuted by an ultracentrifugaldisperser to obtain a polycarboxylic acid compound curing agentcomposition H-4. Its volume mean particle diameter was determined in thesame manner as in Preparation Example 25, which gave the result of 6 μm.Also, the resulting curing agent composition H-4 was dispersed in xyleneto obtain a dispersion (solids content of 30 wt. %) of polycarboxylicacid compound curing agent composition H-4.

Preparation Example 27

Preparation of a Dispersion of a Polycarboxylic Acid Compound CuringAgent Composition H-5

Tetrahydrophthalic anhydride and 1,10-decanedicaboxylic acid, inproportions in solids weight of 5:1, were mixed and charged into aseparable flask where they were caused to melt thermally by heating themunder nitrogen atmosphere to 120° C. The melt was then cooled into asolid form which was subsequently dispersed in xylene and ground by asand grinding mill to provide a dispersion (solids content of 30 wt. %)of a polycarboxylic acid compound curing agent composition H-5. Afraction of the resulting dispersion was collected and placed under areduced pressure to remove xylene therefrom. Its volume mean particlediameter was determined in the same manner as in Preparation Example 25,which gave the result of 6 μm.

Preparation Example 28

Preparation of a Dispersion of a Polycarboxylic Acid Compound CuringAgent Composition H-6

The procedure of Preparation Example 27 was followed, with the exceptionthat tetrahydrophthalic anhydride and 1,10-decanedicaboxylic acid wasmixed in proportions in solids weight of 6:7, to obtain a dispersion(solids content of 30 wt. %) of a polycarboxylic acid compound curingagent composition H-6. A fraction of the resulting dispersion wascollected and placed under a reduced pressure to remove xylenetherefrom. Its volume mean particle diameter was determined in the samemanner as in Preparation Example 25, which gave the result of 6 μm.

Preparation Example 29

Preparation of a Dispersion of a Polycarboxylic Acid Compound CuringAgent Composition H-7

1,10-decanedicarboxylic acid was dispersed in xylene, and ground by asand grinding mill to obtain a dispersion (solids contet of 30 wt. %) ofa polycarboxylic acid compound curing agent composition H-7. A fractionof the resulting dispersion was collected and placed under a reducedpressure to remove xylene therefrom. Its volume mean particle diameterwas determined in the same manner as in Preparation Example 25, whichgave the result of 6 μm.

Preparation Example 30

Comminution of Polycarboxylic Acid Compound Curing Agents

Succinic anhydride and 1,10-decanedicaboxylic acid were individuallycomminuted by an ultracentrifugal disperser to obtain a succinicanhydride curing agent H-8 and a 1,10-decane dicaboxylic acid curingagent H-9. The curing agents H-8 and H-9 were determined for volume meanparticle diameter in the same manner as in Preparation Example 25, whichgave the results of 6 μm, respectively.

EXAMPLE 12

Thermosetting Powder Coating Composition C-15 of the Present Invention

The components specified below were mixed in a sand grinding mill toprepare a raw material solution.

Components Parts by Weight epoxy-containing acrylyc resin R-5 38.5solution (solids content of 65 wt. %) epoxy-containing acrylic resin R-618.5 solution (solids content of 65 wt. %) polyacarboxylic acid compoundcuring agent 43.3 H-3 dispersion (solids content of 30 wt. %) polyesterresin P-1 solution (solids 76.9 content of 65 wt. %) silicone-basedsurface conditioning agent 0.2 acrylic-based surface conditioning agent0.1 UV absorber 1.0 anti-oxidant 1.0 benzoin 0.5

This raw material solution was then added to an aqueous polymercontaining, by weight, 6 parts of GOSENOL GH-20 (polyvinyl alcoholmanufactured by Nippon Gosei Kagaku Co., Ltd., saponification level of88%, no cloud point), 3 parts of GOSENOL KL-05 (polyvinyl alcoholmanufactured by Nippon Gosei Kagaku Co., Ltd., saponification level of80%, cloud point of about 80° C.), 1 part of hydroxypropyl cellulose(cloud point of about 50° C.) and 90 parts of deionized water. Aresulting mixture was further mixed at 25° C. by using a homogenizer toprepare a suspension. The primary oil particles present in thesuspension were measured for a volume mean particle diameter in the samemanner as in Preparation Example 25. The results are given in Table 6.

The suspension was diluted with 300 parts by weight of deionized water,and then transferred to a reaction vessel equipped with a stirrer,thermoregulator, reflux tube and vacuum apparatus.

The suspension was reduced in pressure to 30 Torr. and then heated to35° C. After an interval, the suspension was further reduced in pressureto 140 Torr. and then heated to 60° C. to obtain secondary oilparticles. Subsequently, a solvent present in a dispersed phase wasdistilled off completely to solidify the primary oil particles intoparticles. The resulting suspension was cooled and filtered undersuction to collect particles which were subsequently dried at 30° C.under a reduced pressure to obtain a thermosetting powder coatingcomposition C-15. This powder coating composition C-15 was measured fora volume mean particle diameter in the same manner as in PreparationExample 25. The measurement result is given in Table 6.

EXAMPLES 13-19

Thermosetting Powder Coating Compositions C-16 through C-22 of thePresent Invention

In each Example, the components specified in Table 6 were mixed in asand grinding mill to prepare a raw material solution. Otherwise, theprocedure of Example 12 was repeated to obtain thermosetting powdercoating compositions C-16 through C-22. In each Example, a volume meanparticle diameter was determined for the primary oil particles formed inthe suspension and for the resulting thermosetting powder coatingcomposition. The results are shown in Table 6.

Comparative Examples 4-7

Thermosetting Powder Coating Compositions C-23 through C-26 Prepared bya Dry Process

In each Comparative Example, the components specified in Table 6 weremixed in a Henschel mixer for about 3 minutes and then transferred to aCo-kneader (manufactured by Buss Co., Ltd.) whereby they were meltkneaded at a set temperature of about 95° C. The resulting melt mixturewas cooled to room temperature, crushed by a Henschel mixer and reducedin size by a hammer mill and further by a jet mill. The resulting powderwas classified by a 200-mesh screen to remove coarse particlestherefrom. As a result, comparative thermosetting powder coatingcompositions C-23 through C-26 were obtained. These coating compositionswere measured for volume mean particle diameter in the same manner as inPreparation Example 25. The results are reported in Table 6.

Evaluation Tests

The thermosetting powder coating compositions obtained in Examples 12-19and Comparative Examples 4-7 were evaluated for performances accordingto the following procedures.

A. Smoothness

Each thermosetting powder coating composition was depositedelectrostatically onto an iron plate and baked at 145° C. for 25 minutesto form a coating film having a thickness of 50 μm. The resultingcoating film was measured for center-line average surface roughness (Ra)by using a surface roughness tester (Tokyo Seimitsu Co., Ltd.). Thecoating films having Ra values of not exceeding 0.30 were rated as beingsatisfactory. The results are reported in Table 6.

B. Film Appearance

The coating films formed in the above smoothness test were visuallyevaluated for appearance. The results are given in Table 6. In Table 6,

a rating “⊚” indicates an excellent film appearance,

a rating “∘” indicates a good film appearance, and

a rating “x” indicates a poor film appearance such as hazing.

C. Blocking Resistance

Each thermosetting powder coating composition was stored in a 30° C.incubator for 2 weeks and subsequently subjected to sieving with avibrating 150-mesh screen. The powder coating composition, if a 95% orhigher proportion thereof passed through the screen openings, was ratedas being satisfactory (“∘”). The results are given in Table 6.

TABLE 6 Examples Comparative Examples 12 13 14 15 16 17 18 19 4 5 6 7Coating C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 Coating Epoxy-Resin R6 So- R6 So- R6 So- R6 So- R10 So- R10 So- R6 So- R6 So- R6 R6 R6R6 Formulation Con- A lusion lusion lusion lusion lusion lusion lusionlusion taining 38.5  38.5  38.5  80.0  65.4  23.8  23.8  23.8  22.2 31.2  9.3 9.3 Acrylic Resin R5 So- R5 So- R5 So- R5 So- R9 So- R9 So- R9So- R5 So- R5 So- R5 R5 R5 Resin (a) B lusion lusion lusion lusionlusion lusion lusion lusion lusion 18.5  56.9  80.0  23.8  23.8  86.9 80.0  82.0  15.0  15.0  37.2  31.2  Polycarboxylic H3 H3 H3 H4 H5 H6 H7H7 H8 H8 H9 H4 Acid Compound Disper- Disper- Disper- Disper- Disper-Disper- Disper- Disper- Curing Agent sion sion sion sion sion sion sionsion (b) 43.3  26.7  10.0  85.0  40.0  43.3  75.0  75.0  8.0 3.0 22.5 25.5  Resin (d) Existing P1 So- P1 So- P1 So- P1 So- P2 So- P2 So- R7So- R8 So- P1 So- P1 So- — P1 So- in the Liquid lusion lusion lusionlusion lusion lusion lusion lusion lusion lusion lusion Form at Room76.9  46.2  15.4  15.4  46.2  23.1  15.4  15.4  46.2  15.4  — 9.2Temperature Silicone Surface 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.20.2 Conditioning Agent Acrylic Surface 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 Conditioning Agent UV Absorber 1.0 1.0 1.0 1.0 1.0 1.01.0 1.0 1.0 1.0 1.0 1.0 Anti-Oxidant 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.01.0 1.0 1.0 Benzoin 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5Volume Mean Particle Diameter of 3.8 4.1 3.9 4.2 4.2 3.8 3.7 4.0 — — — —Primary Oil Particles (μm) Thermosetting Volume Mean 17   16   15   15  18   20   15   17   — — 17   — Powder Particle Coating Diameter (μm)Composition Number Mean 8.3 7.0 7.2 7.3 6.9 8.2 8.0 6.3 — — 3.9 —Particle Diameter (μm) Volume to 2.0 2.3 2.1 2.1 2.6 2.4 1.9 2.7 — — 4.4— Number Mean Particle Diameter Evaluation Film Smoothness  0.10  0.22 0.30  0.10  0.09  0.10  0.10  0.10    —*)    —*)  0.41    —*) Tests Ra(μm) Film appearance ∘ ∘ ∘ ∘ ⊚ ⊚ ∘ ∘    —*)    —*) x    —*) Blocking ∘ ∘∘ ∘ ∘ ∘ ∘ ∘    —*)    —*) ∘    —*) Resistance *)Unmeasurable Because ofProduction Failure

As apparent from the results shown in Table 6, the thermosetting powdercoating compositions containing the epoxy-containing acrylic resin (a),polycarboxylic acid compound curing agent (b) and a resin (d) whichtakes a liquid form at room temperature provide coating films havingsatisfactory smoothness, film appearance and blocking resistance.

The following Examples illustrate the third aspect of the presentinvention.

Preparation Example 31

Preparation of an Epoxy-Containing Acrylic Resin R-11

A reaction vessel equipped with a stirrer, thermoregulator and refluxtube was charged with 63 parts by weight of xylene which wassubsequently placed under nitrogen atmosphere and heated to atemperature of 130° C. A mixture containing the below-specifiedcomponents was added dropwise over three hours:

glycidyl methacrylate: 45 parts by weight styrene: 20 parts by weightmethyl methacrylate: 20 parts by weight isobutyl methacrylate: 10 partsby weight 2-hydroxyethyl methacrylate: 5 parts by weight tert-butylperoctoate: 7 parts by weight.

After completion of the dropwise addition, the mixture was held at thesame temperature for additional 3 hours and then cooled to a roomtemperature, so that a solution of an epoxy-containing acrylic resinR-11 (epoxy equivalents of resin solids=180 g/eq, and solidsconcentration=60 wt. %) was obtained. Also, a fraction of the resin R-11solution was heated under a reduced pressure to distill xylene off tothereby provide the resin R-11. Tg of the resulting resin R-11 wasdetermined by DSC 220C (differential scanning calorimeter manufacturedby Seiko Instrument Ind. Co., Ltd., heating rate of 5° C./min.) whichgave the result of 60° C. The measurement according to a turbidimetrymethod revealed an SP value of 10.6. GPC (gel permeation chromatography)revealed a number average molecular weight of 3,500.

Preparation Examples 32-35

Preparation of Epoxy-Containing Acrylic Resins R-12 through R-15

In each Example, the components specified in Table 7 were mixed.Otherwise, the procedure of Example 31 was repeated to obtainepoxy-containing acrylic resin solutions R-12 through R-15. In eachExample, Tg, SP value and number average molecular weight weredetermined in the same manner as in Preparation Example 31. The resultsare shown in Table 7.

Preparation Example 36

Comminuted Curing Agent

1,10-decanedicaboxylic acid was comminuted by an ultracentrifugaldisperser. The comminuted curing agent was measured for volume meanparticle diameter by a coulter counter (Coulter Electronics Co., Ltd.)which gave the result of 6 μm. Also, a melting point of the comminutedcuring agent was determined by a DSC 220C ((manufatured by SeikoInstrument Ind. Co., Ltd., heating rate of 5° C./min.) which gave theresult of 125° C.

Preparation Example 37

Curing Agent Dispersion

1,10-decanedicarboxylic acid was dispersed in xylene, and ground by asand grinding mill to obtain a dispersion (solids content of 30 wt. %)wherein one type of curing agent particles were distributed in xylene.Their volume mean particle diameter was determined by a coulter counter(Coulter Electronics Co., Ltd.) which gave the result of 6 μm.

EXAMPLE 20

Curing Agent Composition H-10

A mixture containing, by weight, 50 parts of 1,10-decanedicaboxylic acidand 50 parts of sebacic acid was comminuted by an ultracentrifugaldisperser to obtain a curing agent composition H-10. A melting point ofthe resulting curing agent composition H-1 was determined by a DSC 220C(manufatured by Seiko Instrument Ind. Co., Ltd., heating rate of 5°C./min.) which gave the result of 120° C. Also, its volume mean particlediameter was determined by a coulter counter (Coulter Electronics Co.,Ltd.) which gave the result of 6 μm.

EXAMPLE 21

Dispersion of a Curing Agent Composition H-11

A mixture containing, by weight, 50 parts of 1,10-decanedicaboxylic acidand 50 parts of sebacic acid was dispersed in xylene and then ground bya sand grinding mill to obtain a curing agent composition H-11dispersion (solids content of 30 wt. %). A fraction of the resultingdispersion was collected and placed uner reduced pressure to removexylene therefrom. The DSC 220C (manufatured by Seiko Instrument Ind.Co., Ltd., heating rate of 5° C/min.) revealed a melting point of 120°C., and the coulter counter (Coulter Electronics Co., Ltd.) revealed avolume mean particle diameter of 6 μm.

EXAMPLE 22

Dispersion of a Curing Agent Composition H-12

50 parts by weight of 1,10-decanedicaboxylic acid and 50 parts by weightof sebacic acid were individually reduced in size by an ultracentrifugaldisperser, mixed in a Henschel mixer, dispersed in xylene and ground bya sand grinding mill to obtain a curing agent composition H-12dispersion (solids content of 30 wt. %). A fraction of the resultingdispersion was collected and placed uner reduced pressure to removexylene therefrom. The DSC 220C (manufatured by Seiko Instrument Ind.Co., Ltd., heating rate of 5° C./min.) revealed a melting point of 120°C., and the coulter counter (Coulter Electronics Co., Ltd.) revealed avolume mean particle diameter of 6 μm.

EXAMPLE 23

Dispersion of a Curing Agent Composition H-13

A mixture containing, by weight, 50 parts of 1,10-decanedicaboxylic acidand 50 parts of sebacic acid was charged into a separable flask where itwas placed under nitrogen atmosphere and thermally melted at 120° C. Themelt was cooled into a solid form which was subsequently dispersed inxylene and ground by a sand grinding mill to obtain a curing agentcomposition H-13 dispersion (solids content of 30 wt. %). A fraction ofthe resulting dispersion was collected and placed under reduced pressureto remove xylene therefrom. The DSC 220C (manufactured by SeikoInstrument Ind. Co., Ltd., heating rate of 5° C./min.) revealed amelting point of 110° C., and the coulter counter (Coulter ElectronicsCo., Ltd.) revealed a volume mean particle diameter of 6 μm.

EXAMPLES 24-27

Dispersions of Curing Agent Compositions H-14 through H-17

In each Example, the formulation specified in Table 8 was used.Otherwise, the procedure of Example 22 was followed to obtaindispersions of curing agent compositions H-14 through H-17. Each curingagent composition dispersion was placed under reduced pressure to removexylene therefrom, and then determined for melting point and volume meanparticle diameter in the same manner as in Example 21. The results aregiven in Table 8. Each curing agent composition was redispersed inxylene to provide dispersions (solids content of 30 wt. %) containingthe curing agent compositions H-14, H-15, H-16 and H-17, respectively.

EXAMPLE 28

Thermosetting Powder Coating Composition C-27 Prepared by a Dry Process

Components Parts by Weight epoxy-containing acrylic resin R-11 70 curingagent composition H-10 20 YF-3919 (polysiloxane based surface 0.53conditioning agent manufactured by Toshiba Silicone Co., Ltd.) benzoin0.45 acrylic surface conditioning agent 0.11 UV absorber 1.2 hinderedamine anti-oxidant 1.0

The above-specified components were blended in a Henschel mixer forabout 3 minutes and melt kneaded at a temperature of about 95° C. by aCo-kneader (manufactured by Buss Co., Ltd.). The resulting melt mixturewas then cooled to room temperature, crushed by a Henschel mixer andreduced in size by a hammer mill and further by a jet mill. Theresulting powder was classified by a 200-mesh screen to remove coarseparticles therefrom, thereby obtaining a thermosetting powder coatingcomposition C-27. The coulter counter (Coulter Electronics Co., Ltd.)revealed a volume mean particle diameter of 12.8 μm and a number meanparticle diameter of 3.1 μm, resulting in the ratio of 4.1.

Comparative Example 8

Thermosetting Powder Coating Composition C-28 Prepared by a Dry Process

The procedure of Example 28 was followed, except that the curing agentcomposition H-10 was replaced by the comminuted curing agent obtained inPreparation Example 36, to obtain a comparative thermosetting powdercoating composition C-28.

The coulter counter (Coulter Elctronics Co., Ltd.) revealed a volumemean particle diameter of 13.5 μm and a number mean particle diameter of3.8 μm, resulting in the ratio of 3.6.

EXAMPLE 29

Thermosetting Powder Coating Composition C-29 Prepared by the WetProcess

Components Parts by Weight epoxy-containing acrylic resin R-11 130solution (resin solids content of 60 wt. %) curing agent compositionH-11 dispersion 75 (solids content of 30 wt. %) YF-3919 (polysiloxanebased surface 0.1 conditioning agent manufactured by Toshiba SiliconeCo., Ltd.) benzoin 0.3 UV absorber 1.2 hindered aniine anti-oxidant 1.0

The above-specified components were blended to provide a raw materialsolution.

This raw material solution was then added to an aqueous polymercontaining, by weight, 6 parts of GOSENOL GH-20 (polyvinyl alcoholmanufactured by Nippon Gosei Kagaku Co., Ltd., saponification level of88%, no cloud point), 3 parts of GOSENOL KL-05 (polyvinyl alcoholmanufactured by Nippon Gosei Kagaku Co., Ltd., saponification level of80%, cloud point of about 80° C.), 1 part of hydroxypropyl cellulose(cloud point of about 50° C.) and 90 parts of deionized water. Theresulting mixture was further mixed at 25° C. by a homogenizer toprepare a suspension which contained primary oil particles having avolume mean particle diameter of 5.0 μm. The resulting dispersion wasdiluted with 300 parts by weight of deionized water, and thentransferred to a reaction vessel equiepped with a stirrer,thermoregulator, reflux tube and vacuum apparatus.

The suspension was reduced in pressure to 30 Torr. and then heated to35° C. After a time interval, the suspension was further reduced inpressure to 140 Torr. and then heated to 60° C., whereby secondary oilparticles were formed. Next, a solvent present in a dispersed phase wasdistilled off completely to solidify the primary oil particles intoparticles. The resulting suspension was cooled and filtered undersuction to collect particles which were subsequently dried at 30° C.under a reduced pressure to obtain a thermosetting powder coatingcomposition C-29. The coulter counter (Coulter Electronics Co., Ltd.)revealed a volume mean particle diameter of 12.3 μm and a number meanparticle diameter of 7.6 μm, resulting in the ratio of 1.6.

Comparative Example 9

Thermosetting Powder Coating Composition C-30 Prepared by the WetProcess

The procedure of Example 29 was followed, except that the curing agentcomposition H-11 dispersion was replaced by the curing agent dispersionobtained in Preparation Example 37, to obtain a comparativethermosetting powder coating composition C-30. The primary oil particleswere found to have a volume mean particle diameter of 3.9 μm. For theresulting powder coating composition, the coulter counter (CoulterElctronics Co., Ltd.) revealed a volume mean particle diameter of 11.1μm and a number mean particle diameter of 8.6 μm, resulting in th ratioof 1.3.

EXAMPLES 30-34

Thermosetting Powder Coating Compositions C-31 through C-35 Prepared bythe Wet Process

The procedure of Example 29 was repeated, except that the curing agentcomposition H-11 dispersion was replaced by the dispersion of curingagent composition H-13, H-14, H-15, H-16 or H-17, to obtainthermosetting powder coating compositions C-31 through C-35. In eachExample, a volume mean particle diameter of the primary oil particlesformed, as well as volume and number mean particle diameters of theresulting powder coating composition, were determined in the same manneras in Example 28. The ratio of the volume to number mean particlediameter was also calculated from therefrom. The results are shown inTable 9.

EXAMPLE 35

Thermosetting Powder Coating Composition C-36 Prepared by the WetProcess

The procedure of Example 29 was followed, except that 130 parts byweight of the epoxy-containing acrylic resin R-11 solution was alteredto a mixture of 100 parts by weight of the epoxy-containing acrylicresin R-12 solution and 30 parts by weight of the epoxy-containingacrylic resin R-13 solution and that the curing agent composition H-11dispersion was replaced by the curing agent composition H-14 dispersion,to obtain a thermosetting powder coating composition C-36. The primaryoil particles produced were determined to have a volume mean particlediameter of 4.5 μm. For the resulting powder coating composition, thecoulter counter (Coulter Elctronics Co., Ltd.) revealed a volume meanparticle diameter of 9.5 μm and a number mean particle diameter of 6.9μm, resulting in the ratio of 1.4.

EXAMPLE 36

Thermosetting Powder Coating Composition C-37 Prepared by the WetProcess

The procedure of Example 29 was followed, except that 130 parts byweight of the epoxy-containing acrylic resin R-11 solution was alteredto a mixture of 70 parts by weight of the epoxy-containing acrylic resinR-14 solution and 60 parts by weight of the epoxy-containing acrylicresin R-15 solution and that the curing agent composition H-11dispersion was replaced by the curing agent composition H-14 dispersion,to obtain a thermosetting powder coating composition C-37. The primaryoil particles produced were determined to have a volume mean particlediameter of 4.1 μm. For the resulting powder coating composition C-37,the coulter counter (Coulter Elctronics Co., Ltd.) revealed a volumemean particle diameter of 11.2 μm and a number mean particle diameter of8.9 μm, resulting in the ratio of 1.3.

Comparative Example 10

Thermosetting Powder Coating Composition C-38 Prepared by the WetProcess

The procedure of Example 35 was followed, except that the curing agentcomposition H-14 dispersion was replaced by the curing agent dispersionobtained in Preparation Example 37, to obtain a thermosetting powdercoating composition C-38. The primary oil particles produced weredetermined to have a volume mean particle diameter of 4.1 μm. For theresulting powder coating composition, the coulter counter (CoulterElctronics Co., Ltd.) revealed a volume mean particle diameter of 10.5μm and a number mean particle diameter of 7.5 μm, resulting in the ratioof 1.4.

Comparative Example 11

Thermosetting Powder Coating Composition C-39 Prepared by the WetProcess

The procedure of Example 36 was followed, except that the curing agentcomposition H-14 dispersion was replaced by the curing agent dispersionobtained in Preparation Example 37, to obtain a thermosetting powdercoating composition C-39. The primary oil particles produced weredetermined to have a volume mean particle diameter of 4.6 μm. For theresulting powder coating composition, the coulter counter (CoulterElctronics Co., Ltd.) revealed a volume mean particle diameter of 10.3μm and a number mean particle diameter of 7.5 μm, resulting in the ratioof 1.4.

Table 9 lists the volume mean particle diameters of primary oilparticles, the volume mean and number mean particle diameters ofthermosetting powder coating compositions and the ratios of the volumeto number mean particle diameter, respectively obtained in the aboveExamples 28-36 and Comparative Examples 8-11.

Evaluation Tests

The thermosetting powder coating compositions obtained in Examples 28-36and Comparative Examples 8-11 were evaluated for performances accordingto the following procedures. The results are reported in Table 9.

A. Visual Examination of a Film Appearance

Each thermosetting powder coating composition was appliedelectrostatically onto an iron plate and baked at 145° C. for 25 minutesto form a coating film having a thickness of 60 μm. The resultingcoating film was visually observed for its appearance. In Table 9, thecoating film, unless accompanied by a hazing phenomenon, is indicated bya rating of “∘”, and, if accompanied, by a ratio of “x”,

B. Smoothness

1) Smoothness of a coating film on an iron plate:

An appearance of the coating film formed on the iron plate in the above.test was evaluated in terms of an NSIC value (%) when determined by animage sharpness measuring apparatus (manufactured by Suga testingmachine Co., Ltd.). The coating films having an NSIC value of 65% orhigher were rated as being satisfactory.

2) Smoothness of a coating film on a water-borne basecoat:

SUPERLAC M260-SILVER (water-borne metallic basecoat manufactured byNippon Paint Co., Ltd.) was applied electrostatically on an intercoatedsubstrate to a dry thickness of 10-20 μm, and provisionally heated in ahot-air oven controlled at 80° C. for 10 minutes. After the substratewas cooled to room temperature, each thermosetting powder coatingcomposition was applied electrostatically on the basecoat to a thicknessof 50 μm and baked in the hot-air drying oven controlled at 145° C. for25 minutes. After completion of baking, the substrate was taken out fromthe oven and left to stand. As the substrate temperature dropped toambient, the smoothness of the resulting coating film was evaluated interms of an NSIC value (%) as measured by an image sharpness measuringapparatus (manufactured by Suga testing machine Co., Ltd.). The coatingfilms having an NSIC value of 60% or higher were rated as beingsatisfactory.

The aforementioned intercoated substrate was prepared byelectrodepositing POWERTOP U-50 (automobile-use cationicelectrodeposition coating manufactured by Nippon Paint Co., Ltd.) onto azinc phosphated dull steel plate to a dry thickness of about 25 μm,baking the deposited layer at 160° C. for 30 minutes, electrostaticallydepositing ORGA P-2 (intercoat composition manufactured by Nippon PaintCo., Ltd.) onto the undercoat to a dry thickness of about 40 μm, andbaking the applied layer at 140° C. for 30 minutes.

C. Storage Stability

Each thermosetting powder coating composition was stored in a 30° C.incubator for 2 months and subsequently subjected to sieving with avibrating 150-mesh screen. The coating composition, if a 95% or higherproportion thereof passed through the screen openings, was found asbeing satisfactory.

D. Resistance to Solid-Phase Reaction

Each thermosetting powder coating composition was stored in a 30° C.incubator for 2 months, and the rate of its change in weight averagemolecular weight was determined by GPC. It was also subjected to theabove-described smoothness test on an iron plate, and the resultingcoating film, if exhibited an NSIC value of 65% or higher, was rated asbeing satisfactory.

TABLE 7 Preparation Examples 31 32 33 34 35 Epoxy-Containing AcrylicResin R11 R12 R13 R14 R15 Components Styrene 20 20 20 20 20 MethylMethacrylate 20 — 30 — 20 Glycidyl Methacrylate 45 45 45 60 60 IsobutylMethacrylate 10 5 5 — — 2-Hydroxyethyl Methacrylate — 30 — — — n-ButylMethacrylate — — — 15 — n-Butyl Acrylate — — — 5 — t-Butyl Peroctoate 73 10 5 7 Characteristic Resin Solids Epoxy Equivalent (g/eq) 180 180 180240 240 Values Tg (° C.) 60 30 70 40 70 SP Value 10.6 10.0 10.4 10.510.9 Number Average Molecular Weight 3500 9000 3000 5000 3500

TABLE 8 Preparation Examples Example Dispersion of Curing 21 22 23 24 2526 27 36 Agent Composition H11 H12 H13 H14 H15 H16 H17   —*) Components1,10-Decane Dicarboxylic Acid 50 50 50 75 75 50 75 100 Sebacic Acid 5050 50 25 20 — — — Butane Tricarboxylic Acid — — — — 5 — 5 — HexanediolDisuccinate — — — — — 50 20 — Measured Melting Point (° C.) 120 120 110110 108 96 95 125 Values Volume Mean Particle Diameter (μm) 6 6 6 6 6 66 6 *)Comminuted Curing Agent Only DSC-Determined Melting Points ofComponents: M.P. of 1,10-Decane Dicarboxylic Acid = 125° C. M.P. ofSebacic Acid = 130° C. M.P. of Butane Tricarboxylic Acid = 119° C. M.P.of Hexanediol Disuccinate = 101° C.

TABLE 9 Examples Comparative Examples 28 29 30 31 32 33 34 35 36 8 9 1011 Primary Oil Volume Mean Particle 12.8 5.0 4.8 4.2 3.8 4.4 4.2 4.5 4.113.5 3.9 4.1 4.6 Particles Diameter (μm) Secondary Oil Volume MeanParticle — 12.3 10.3 10.1 13.0 15.0 14.4 9.5 11.2 — 11.1 10.5 10.3Particles Diameter (μm) Thermosetting Volume Mean Particle 12.8 12.310.3 10.1 13.0 15.0 14.4 9.5 11.2 13.5 11.1 10.5 10.3 Powder CoatingDiameter (μm) Composition Number Mean Particle 3.1 7.6 7.1 7.5 7.8 10.910.6 6.9 8.9 3.8 8.6 7.5 7.5 Diameter (μm) Volume to Number Mean 4.1 1.61.5 1.3 1.7 1.4 1.4 1.4 1.3 3.6 1.3 1.4 1.4 Particle Diameter VisualObservation ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ x x x x Smoothness on an Iron Plate 70 7880 75 79 78 75 75 81 52 50 48 49 (NSIC %) on an Aqueous Basecoat — — — —— — — 67 63 — — 39 40 Storage Mesh Pass Rate (%) 30 97 98 98 98 97 98100 100 38 84 100 100 Stability Solid Reaction Mw*) Change Rate (%) 1.401.02 1.01 1.01 1.02 1.02 1.01 1.01 1.01 1.37 1.02 1.02 1.02 ResistanceNSIC (%) 69 76 79 74 78 77 74 74 80 51 49 47 48 *)Weight AverageMolecular Weight

The thermosetting powder coating compositions according to the firstaspect of the present invention, because of its inclusion of thethermosetting resin, curing agent and fine crosslinked resin particlesand its production by the wet process, exhibit improved storagestability and, when applied onto a substrate and heated, result incoating films which show little occurrence of sagging as a result ofmelt flow of the composition on vertical or curved surface portions ofthe substrate and which are excellent in smoothness and appearance. Thisis considered due to the inclusion of fine crosslinked resin particleswhich, when applied and heated, act on each other to develop astructural viscosity.

The thermosetting powder coating compositions according is to the secondaspect of the present invention, because of its inclusion of theepoxy-containing acrylic resin, poly-carboxylic acid compound curingagent and resin which exists in the liquid form at room temperature andits production by the wet process, exhibit improved storage stabilityand can provide coating films excellent in smoothness. This isconsidered due to the inclusion of the room temperature liquid-formresin which, when applied and heated, serves to reduce a melt viscosityto result in the improved fluidity. Another attributing factor may bethe limited application of heat, during production of the coatingcompositions, which suppresses solid-phase reactions between theformulation components.

In the first and second aspects of the present invention, theincorporation of the particular resins A and B different in type fromeach other in the aforementioned epoxy-containing acrylic resin furtherimproves the storage stability of coating compositions and smoothness ofresulting coating films. During the wet process production of powdercoating compositions, the resin A having a higher Tg and SP value isbelieved to exist at a shell wall of a powder particle surrounded bydispersing water in the increased concentration compared to the resin B,resulting in the increased Tg of the particle shell wall relative to theparticle interior. This is considered to have provided the improvedblocking resistance to the thermosetting powder coating compositions ofthe present invention. Also, the overall Tg of the thermosetting powdercoating composition is made lower than the Tg of the particle shellwalls. This is considered to further improve the smoothness of resultingcoating films.

In the particular cases where the polycarboxylic acid compound curingagent includes the two types of compounds and shows the specificbehaviors, the resulting coating films have the improved appearance.This is considered due to the increased compatibility of a whole systemas a result of the reduced cohesive force of the curing agent. Also,since the melting point of the polycarboxylic acid compound curing agentcan be adjusted to lower values, the heating temperature can also bemade lower.

The thermosetting powder coating compositions including the curing agentcomposition according to the third aspect of the present inventionresult in coating films which present good appearance and show thereduced occurrence of a hazing phenomenon. The use of the curing agentcomposition of the present invention is considered to decrease thecuring agent cohesiveness so that the curing agent composition, when thecoating composition is applied and thermally melted, serves to improvethe compatibility of a whole system.

Also, the curing agent composition according to the third aspect of thepresent invention shows a DSC absorption curve different from the curvesfrom pure components, i.e., from the polycarboxylic acid compound (b-1)and carboxylic acid compound (b-2), and, when applied and allowed tothermally melt, exhibits a unique behavior different from that of asimple mixture of the two pure components. This is considered to haveled to the improvement in appearance of resulting coating films.

The further improvement in appearance of coating films can be achievedif the curing agent composition is prepared by mixing the polycarboxylicacid compound (b-1) and carboxylic acid compound (b-2), both in theliquid form. This is considered due to the improved cohesion of theresulting curing agent composition. That is, the both compounds arebelieved to be intermingled more sufficiently on a molecular level whenthey are mixed in the liquid form than when they are mixed in theconventional form.

For the curing agent composition of the present invention, its meltingpoint can be made lower. With the use of this low-melting curing agentcomposition, thermosetting powder coatings containing an acid/epoxycuring system can be rendered low-temperature curable.

The thermosetting powder coating composition of the present inventionresult in a coating film with least occurrence of a hazing phenomenon.This permits high loading of the curing agent composition, leading tothe improved crosslinking density of the resulting coating film.

The thermosetting powder coating composition of the present inventionhas a narrow particle size distribution with particles being uniform inshape. It accordingly contains an extremely small quantity of dust and,when recovered, can be reused like fresh coatings. It also exhibitsimproved coating operation characteristics such as transportability andtransfer efficiency. Furthermore, when applied in the form of a thinfilm, it results in a smooth coating film.

As a result of using the aforementioned thermosetting powder coatingcomposition, the practice of the multilayer film forming method of thepresent invention results in the provision of coating films excellent insmoothness. Therefore, the multilayer film forming method of the presentinvention can be advantageously applied to automotive bodies to providesmooth multilayer films thereon.

What is claimed is:
 1. A thermosetting powder coating compositionprepared by a wet process, said composition containing a mixturecomprising: (a) epoxy-containing acrylic resin A and epoxy-containingacrylic resin B, said resins A and B satisfying the properties: (1)(SP_(A)-SP_(B)) is within the range of from 0.2 to 1.5, wherein SP_(A)is the solubility parameter of resin A and SP_(B) is the solubilityparameter of resin B; (2) T_(g)(A)-T_(g)(B)≧10° C., wherein T_(g)(A) isthe glass transition temperature of resin A and T_(g)(B) is the glasstransition temperature of resin B; (3) T_(g)(A) is within the range offrom 40 to 100° C. and T_(g)(B) is within the range of from 20 to 50°C.; and (4) a solids weight ratio of resin A to resin B is within therange of from 5:95 to 50:50; (b) a polycarboxylic acid compound curingagent; and (c) fine crosslinked resin particles. (d) a resin in liquidform in an amount of from 5-70 parts by solids weight per 100 parts ofthe total solids weight of epoxy-containing resin (a) and resin (d),wherein (SP_(B)-SP_(d)) is within the range of from 0.01 to 1.5, SP_(d)being the solubility parameter of resin (d); said wet process comprisingthe steps of: (1) allowing components (a), (b), (c) and (d) to dissolveor disperse in an organic solvent to provide a solution; (2) adding saidsolution to an aqueous solution containing a water-soluble polymerhaving a cloud point of from 30 to 90° C. and mixing them at atemperature below said could point to provide a suspension containingprimary oil particles; and (3) heating said suspension to a temperatureequal to or higher than the cloud point to form secondary oil particlesand distilling off said organic solvent from a system to collect heparticles.
 2. The thermosetting powder coating composition of claim 1,wherein said organic solvent is distilled off from a system in said step(2).
 3. The thermosetting powder coating composition of claim 1, whereinsaid resin (d) has a number average molecular weight of 200-5,000. 4.The thermosetting powder coating composition of claim 1, wherein saidresin (d) is at least one selected from the group consisting of apolyester resin, an acrylic resin, a polyol and a polyepoxy.
 5. Thethermosetting powder coating composition of claim 1, wherein said resin(d) has a functional group curable with said epoxy-containing acrylicresin (a) or said polycarboxylic acid compound curing agent (b).
 6. Thethermosetting powder coating composition of claim 1, wherein saidthermosetting powder coating composition is a thermosetting powder clearcoating composition.